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Occultation

An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks from view (occults) an object in the background. In this general sense, occultation applies to the visual scene observed from low-flying aircraft (or computer-generated imagery) when foreground objects obscure distant objects dynamically, as the scene changes over time.

If the closer body does not entirely conceal the farther one, the event is called a transit. Both transit and occultation may be referred to generally as occlusion; and if a shadow is cast onto the observer, it is called an eclipse.

The symbol for an occultation, and especially a solar eclipse, is (U+1F775 🝵).

Occultations by the Moon

The term occultation is most frequently used to describe lunar occultations, those relatively frequent occasions when the Moon passes in front of a star during the course of its orbital motion around the Earth. Since the Moon, with an angular speed with respect to the stars of 0.55 arcsec/s or 2.7 μrad/s, has a very thin atmosphere and stars have an angular diameter of at most 0.057 arcseconds or 0.28 μrad, a star that is occulted by the Moon will disappear or reappear in 0.1 seconds or less on the Moon's edge, or limb. Events that take place on the Moon's dark limb are of particular interest to observers, because the lack of glare allows easier observation and timing.

The Moon's orbit is inclined slightly with respect to the ecliptic (see orbit of the Moon) meaning any star with an ecliptic latitude between –6.6 and +6.6 degrees may be occulted by it. Three first magnitude stars appear well within that band – Regulus, Spica, and Antares – meaning they may be occulted by the Moon or by planets. Occultations of Aldebaran are in this epoch only possible by the Moon, because the planets pass Aldebaran to the north. Neither planetary nor lunar occultations of Pollux are currently possible, however several thousand years ago lunar occultations were possible. Some notably close deep-sky objects, such as the Pleiades, can be occulted by the Moon.

Within a few kilometres of the edge of an occultation's predicted path, referred to as its northern or southern limit, an observer may see the star intermittently disappearing and reappearing as the irregular limb of the Moon moves past the star, creating what is known as a grazing lunar occultation. From an observational and scientific standpoint, these "grazes" are the most dynamic and interesting of lunar occultations.

The accurate timing of lunar occultations is performed regularly by (primarily amateur) astronomers. Lunar occultations timed to an accuracy of a few tenths of a second have various scientific uses, particularly in refining our knowledge of lunar topography. Photoelectric analysis of lunar occultations have also discovered some stars to be very close visual or spectroscopic binaries. Some angular diameters of stars have been measured by timing of lunar occultations, which is useful for determining effective temperatures of those stars. Early radio astronomers found occultations of radio sources by the Moon valuable for determining their exact positions, because the long wavelength of radio waves limited the resolution available through direct observation. This was crucial for the unambiguous identification of the radio source 3C 273 with the optical quasar and its jet, and a fundamental prerequisite for Maarten Schmidt's discovery of the cosmological nature of quasars.

Several times during the year the Moon can be seen occulting a planet. Since planets, unlike stars, have significant angular sizes, lunar occultations of planets will create a narrow zone on Earth from which a partial occultation of the planet will occur. An observer located within that narrow zone could observe the planet's disk partly blocked by the slowly moving Moon. The same mechanism can be seen with the Sun, where observers on Earth will view it as a solar eclipse. Therefore, a total solar eclipse is essentially the Moon occulting the Sun.

Occultation by planets

Stars may also be occulted by planets. Occultations of bright stars are rare. In 1959, Venus occulted Regulus, and the next occultation of a bright star (also Regulus by Venus) will be in 2044. Uranus's rings were first discovered when that planet occulted a star in 1977. On 3 July 1989, Saturn passed in front of the 5th magnitude star 28 Sagittarii. Pluto occulted stars in 1988, 2002, and 2006, allowing its tenuous atmosphere to be studied via atmospheric limb sounding.

In rare cases, one planet can pass in front of another. If the nearer planet appears larger than the more distant one, the event is called a mutual planetary occultation. The last occultation or transit occurred on 3 January 1818 and the next will occur on 22 November 2065, in both cases involving the same two planets—Venus and Jupiter.

Jupiter rarely occults Saturn. This is one of the rarest events known, with the next occurrence on February 10, 7541. This event is visible worldwide since the duo would be positioned almost in opposition to the sun, in the border line between the constellations of Orion and Taurus. In some areas this occultation cannot be seen, but when viewed through even small telescopes, both gas giants appear to be in the same part of view through the eyepiece. The last one occurred in 6857 B.C.E.

Occultations by smaller bodies
A further set of occultations are those when a small Solar System body or dwarf planet passes in front of a star, temporarily blocking its light as seen from Earth. These occultations are useful for measuring the size and position of body much more precisely than can be done by other means. A cross-sectional profile of the shape of a body can even be determined if a number of observers at different, nearby, locations observe the occultation. Occultations have been used to calculate the diameter of trans-Neptunian objects such as, Ixion and Varuna. Software for coordinating observations is available for download at http://www.occultwatcher.net/

In addition, mutual occultation and eclipsing events can occur between a primary and its satellite. A large number of moons have been discovered analyzing the photometric light curves of small bodies and detecting a second, superimposed brightness variation, from which an orbital period for the satellite (secondary), and a secondary-to-primary diameter-ratio (for the binary system) can often be derived.

Asteroids

 * On 29 May 1983, 2 Pallas occulted the naked-eye bright spectroscopic binary star 1 Vulpeculae along a track across the southern United States, northern Mexico, and north parts of the Caribbean. Observations from 130 different locations defined the shape of about two-thirds of the asteroid, and detected the secondary companion of the bright binary star; these observations, taken together with those of a separate occultation by Pallas in 1979, provided a complete figure for the asteroid eight years before any asteroid was visited by a spacecraft (the much smaller Gaspra by Galileo in October 1991).
 * On 12 March 2009 nine asteroids (85 Io, 247 Eukrate, 1585 Union, 201 Penelope, 70 Panopaea, 980 Anacostia, 2448 Sholokhov, 1746 Brouwer, and 191 Kolga) occulted notable magnitude stars, viewed from given places on the Earth.
 * According to the 1998 European Asteroidal Occultation Results from Euraster, 39 Laetitia was observed by over 38 observatories in one occultation on 3 March 1998, which resulted in many chords being determined.
 * The star Regulus was occulted by the asteroid 163 Erigone in the early morning of 20 March 2014. This was the brightest occultation of an asteroid ever predicted to occur over a populated area. As the main belt asteroid passed in front of the star its 100 km shadow swept across Nassau and Suffolk counties, all of New York City and the Hudson River Valley, with the center of the shadow path following a line roughly connecting New York City, White Plains, Newburgh, Oneonta, Rome, and Pulaski before crossing into Canada near Belleville and North Bay, Ontario. Bad weather obscured the occultation.

Distant objects

 * Preliminary results of a 6 November 2010 occultation by the dwarf planet Eris of a magnitude 17 star in the constellation of Cetus placed an upper limit on Eris's diameter of 2320 km, making it almost the same size as Pluto. Due to their slower movement through the night sky, occultations by TNOs are far less common than by asteroids in the main-belt.
 * The dwarf planet Haumea was observed in a stellar transit on 21 January 2017, identifying a ring.
 * On 3 June 2017, a star was occulted by the Kuiper belt object 486958 Arrokoth, the first such occultation detected. The multi-faceted campaign involved cooperation from the Argentinian government (including local governments – a major highway was closed for two hours, and street lights were turned off, in order to preclude light pollution), three spacecraft, 24 portable ground-based telescopes, and NASA's SOFIA airborne observatory in "the most challenging stellar occultation in the history of astronomy," in an effort spanning six months.

Double occultations
The Moon or another celestial body can occult multiple celestial bodies at the same time.

Because of its relatively large angular diameter the Moon, at any given time, occults an indeterminate number of stars and galaxies. However the Moon occulting (obscuring) two bright objects (e.g. two planets or a bright star and a planet) simultaneously is extremely rare and can be seen only from a small part of the world: the last such event was on 23 April 1998 when it occulted Venus and Jupiter for observers on Ascension Island.

Artificial occultations
The Big Occulting Steerable Satellite (BOSS) was a proposed satellite that would work in conjunction with a telescope to detect planets around distant stars. The satellite consists of a large, very lightweight sheet, and a set of maneuvering thrusters and navigation systems. It would maneuver to a position along the line of sight between the telescope and a nearby star. The satellite would thereby block the radiation from the star, permitting the orbiting planets to be observed.

The proposed satellite would have a dimension of 70 x, a mass of about 600 kg, and maneuver by means of an ion drive engine in combination with using the sheet as a light sail. Positioned at a distance of 100,000 km from the telescope, it would block more than 99.998% of the starlight.

There are two possible configurations of this satellite. The first would work with a space telescope, most likely positioned near the Earth's L2 Lagrangian point. The second would place the satellite in a highly elliptical orbit about the Earth, and work in conjunction with a ground telescope. At the apogee of the orbit, the satellite would remain relatively stationary with respect to the ground, allowing longer exposure times.

An updated version of this design is called the Starshade, which uses a sunflower-shaped coronagraph disc. A comparable proposal was also made for a satellite to occult bright X-ray sources, called an X-ray Occulting Steerable Satellite or XOSS.

title: Occultation


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Talk:Sergio Leone

Unedited version of movie and pronunciation of director
The author mentioned that "Once Upon a Time in the West" was edited. Is it possible to see an unedited version of this film? Thanks- ARog

Is his name pronounced "Lee - Own" or "lee - own - ee"?

 * Approximately "le oh ne". - Vague | Rant 08:03, 17 March 2005 (UTC)

Painter
i have a painting that is signed by a leone, it is a very old painting. Do you know if this talented man painted. thank you smilinda@rogers.com

If the painting is "very old" it was probably not painted by someone who died less than 20 years ago. Also, there are most likely many Leones in the world. (The more notable Leones are found on this page: Leone (disambiguation).) A cursory Google search turns up no mentions of Sergio Leone being a painter. Good luck with your search. Menthaxpiperita (talk) 02:24, 27 February 2008 (UTC)

"One of the greatest directors"
Paragraph 1: It is a fact that Leone is widely considered to be one of the greatest directors of all time, not just one of the greatest Western directors - People who edit pages should not be totally ignorant of the subject matter they are editing!!!!!!!!!! Yes, he made mainly Westerns, but his film, "Once Upon a Time in America" is also widely acknowledged as a masterpiece! It is not as if his acknowledged directorial skills were only evident in his Westerns! The piece already acknowledges that he was most famous for his Spaghetti Westerns!

 * Hello, 82.26.184.39. It wasn't so much the "considered to be one of the greatest directors of all time" part I thought needed reverting as the "if not the greatest!" that followed it. This is supposed to be an encyclopedia article, and a hyperbolic statement like that, complete with an exclamation point, does not sound very encyclopedic. Rizzleboffin 23:50, 22 October 2006 (UTC)

Thanks Rizz - fair enough - just that my new version was reverted too - and I think the change was necessary in order to accurately reflect his status!

I have deleted the sentence mentioned above because it added nothing to the article. Use of Peacock terms is discouraged in wikipedia articles, especially in the lead section. You should show the reader why this subject is relevant (mention awards, cultural impact, etc) using verifiable sources, rather than simply saying it is important. 168.28.237.131 (talk) 01:50, 27 February 2008 (UTC)

Restructure
The entire text of this article is practically under "biography," but that can be broken up into smaller sections.--75.30.181.232 07:55, 22 November 2006 (UTC)

Influences/influenced
In section 1.4 (Death and later years), there's a huge list of directors who have been influenced by him. I think it would be easier to just add an "influenced" section to the infobox. Comments? Questions? MwNNrules (talk) 20:31, 19 May 2008 (UTC)

In the Amazon reviews of Johnny Guitar, a contributor states that this film had a significant influence on Leone's style. Can anyone confirm or deny? WilliamSommerwerck (talk) 12:58, 5 December 2010 (UTC)

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 * Sergio Leone.jpg

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 * Sergio Leone and Robert De Niro.png

Comments on the article
1. The section on the 1960s goes in detail talking about A Fistful of Dollars as if it was his best movie but barely mentions For a Few Dollars More and The Good, the Bad and the Ugly. The last movie should have a paragraph on its own.

2. Bianco, rosso e Verdone and Troppo Forte are listed under the section on the 1970s but they are movies from the 1980s so they are out of place.

3. The image in the 1980s section is obviously a photo of Sergio Leone. It would be appropriate to add a description that indicates when it was taken or anything useful about it.

4. The caption that says "Grave of Sergio Leone" is not showing the grave of the director.

5. "Critical reception to Leone's directorial features."

The above is not a complete sentence.

ICE77 (talk) 07:34, 12 February 2023 (UTC)

Private life
Although I do not like to pry I was still surprised that this article has no mention of Sergio Leone's private life. Is it really so difficult to dig up reliable references on the fact that he was married (I believe to Carla Ranalli) and had three children (I believe Andrea, Francesca and Raffaella)? They do not matter in the fact that he is considered a great director but Andrea is mentioned (at time I am writing this), so it would make sense to have at least some background. Also I wonder why no mention is made of the fact that he directed For a Fistful of Dollars under the pseudonym Bob Robertson, an obvious reference to his father, who some believe made the first "Spaghetti Western" (starring his mother). John Do'h 13:28, 26 September 2023 (UTC)

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Talk:Sergio Leone


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Open-water diving

In underwater diving, open water is unrestricted water such as a sea, lake, river, or flooded quarry. It is a contradistinction to confined water (usually a swimming pool) where initial skills training takes place. Open water also means the diver has direct vertical access to the surface of the water in contact with the Earth's atmosphere. Open water diving implies that if a problem arises, the diver can directly ascend vertically to the atmosphere to breathe air. Penetration diving—involving entering caves or wrecks, or diving under ice—is therefore not "open water diving". In some contexts the lack of a decompression obligation is considered a necessary condition for classification of a dive as an open water dive, as a decompression obligation is a procedural and safety restriction on immediate ascent to the surface, but this does not affect the classification of the venue as open water.

Swim-throughs – the recreational diving term for arches and short, clear, tunnels where the natural light can be seen at the far end, and there is enough clearance that it is theoretically possible for the diver to pass through the narrowest point without contact with the sides, bottom or ceiling, are technically an overhead environment, but this is often overlooked by divers as there is no risk of getting lost inside, and the risk of entrapment is generally low.

Divers progress from learning diving skills in confined or benign water such as a swimming pool to practicing skills in open water in which the environment is not restricted to a small, controlled locality and depth, with conditions more typical of a natural body of water which might be used by divers, and the range of hazards and associated risk is significantly expanded. In this context confined water and benign water are special cases of open water, as they comply with the more general condition of unobstructed access to the surface.

Some recreational diver certification agencies use a variation on this term in the title of their entry level certification. Open Water Diver certification implies that the diver is competent to dive in unrestricted water, with various constraints regarding the conditions, and particularly that their competence is limited to diving in open water with free access to the surface.

Open ocean diving
The extreme case of open water is the deep open ocean, where the bottom is at a depth which is irrelevant to the diver as there would be no chance of surviving long enough to reach it. Open oceanic water is often remarkably clear, but this is not always the case. There is no natural visual reference for depth in the open ocean, and depth monitoring and control is critically important to diver safety.

Blue-water diving
Blue-water diving is done in mid-water where the bottom is out of sight of the diver and there may be no fixed visual reference. It is used by scientific divers for direct observation and sampling of pelagic organisms and particulate matter, particularly the gelatinous zoo-plankton that are fragile and transparent, making them relatively inaccessible by other methods, and by recreational divers for observation and photography of a range of organisms not easily seen in inshore waters.

The techniques of blue-water diving have been developed over the years to suit the conditions and address the hazards of an environment which is functionally bottomless, and has no fixed visible positional references. The diver who is focused on small organisms or instruments at close range is likely to have diminished awareness of depth, buoyancy, current, surge, other divers, large organisms, and even the direction to the surface.

An accepted procedure for scientific blue-water collection diving with several working divers, is to tether the working divers to a central hub connected to a surface platform, and to have an in-water safety diver attend the hub. The tethers pass through fairleads at the hub and are tensioned by a weight at the end, which keeps slack out of the line and thereby reduces the risk of entanglement, and prevents the end of the line from passing through the fairlead. The tether serves to limit the distance a diver can move away from the hub, which is typically fastened to a substantial downline supported by a large buoy at the surface, and kept vertical by a weight. The float at the surface allows the divers to move freely in the water column within the constraint of the tether, and drift with the current. The tethers also allow rope signals between the safety diver and the working divers. The surface platform, generally a small boat, may be tethered to the buoy, and if there is sufficient wind to make this a problem a parachute sea anchor can be deployed to minimise drift. Windage will generally position the buoy and boat downwind of the parachute. The other end of the tether is clipped to the diver's harness or buoyancy compensator by some form of quick-release shackle. These procedures and equipment can also be used at night.

If a sea anchor is deployed to limit drift, it must be kept clear of the divers to minimise the risk of entanglement, and it should be buoyed to prevent sinking in a calm. The sea anchor cable should be buoyant line for the same reason. Blue-water diving operations are constrained by water and weather conditions, including wind, sea state, current strength, visibility, and the presence of aggressive predators.

Black-water diving and blackwater photography
Black-water diving is mid-water diving at night, particularly on a moonless night. The environment is referred to as black-water. The term black-water may also be used to refer to diving in zero visibility, or in sewage. Christopher Newbert, author of Within a Rainbow Sea, (1984) is credited as an early black-water diver. In his book, he describes solo offshore night dives to depths of up to 150 ft. Black-water diving is often done as a photographic opportunity for recreational divers as there can be a wide range of plankton that would not often be seen by day or closer inshore. This is known as blackwater photography.

Weighted downlines are commonly used to provide a stable vertical reference. These may be tied to the boat or supported by a buoy. Each diver may be attached to a downline using a shorter tether to ensure that divers do not go too far from the boat or too deep. The downline may be marked with lights to indicate depth and to attract mobile organisms. A wide range of animal life may be seen, including many species that spend the daylight hours at depths below those accessible to ambient pressure divers, and migrate vertically through the water column on a diurnal cycle. Many of these are bioluminescent or translucent or both.

The boat moves differently from the divers and the divers move differently from the plankton, making it necessary to work to get a position from which there will be enough time to frame and take photos of the drifting subjects. The boat will drift under the influence of current and wind, while the divers and plankton will drift with the current. If there is wind the boat will drag the downlines, and if tethered to the buoy, it will drag the buoy and divers attached. If divers swim ahead of the towing action, they will have a short time to drift with the plankton and take photos until the slack in the line has been taken up, at which point the plankton will be left behind. This drift problem can be reduced by setting a parachute anchor, which will reduce wind drift to a negligible amount. The divers should stay clear of the entanglement hazard of the parachute suspension lines.

title: Open-water diving


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Gran Paradiso

The Gran Paradiso or Grand Paradis  is a mountain in the Graian Alps in Italy, located between the Aosta Valley and Piedmont regions. It is located in Gran Paradiso National Park.

Etymology
The Italian toponym "Gran Paradiso" derives, via the French Grand paradis, from the Aosta Valley patois Granta Parey, which means large wall. It is the same etymology as nearby Granta Parey.

Geography
The peak is the 7th highest mountain in the Graian Alps, with an elevation of 4,061 m. In the SOIUSA (International Standardized Mountain Subdivision of the Alps) the mountain belongs to an alpine subsection called "North-Eastern Graian Alps" (It:Alpi del Gran Paradiso; Fr:Alpes du Grand-Paradis) and also gives its name to the gruppo del Gran Paradiso.

While the Mont Blanc massif straddles the border between France and Italy, the Gran Paradiso is the only mountain whose summit reaches over 4,000 metres that is entirely within Italian territory.

Routes
Climbs normally start from either the Refuge Frédéric Chabod or the Refuge Victor-Emmanuel II. The latter is named after Victor Emmanuel II of Italy who created the Gran Paradiso royal reserve in 1856, presently the site of the Gran Paradiso National Park.

It is widely accepted that Gran Paradiso is one of the easiest four-thousanders to summit. This is not fully true, however, because while almost the entire route to the ridge is graded at F+, the last several dozen metres to the Virgin Mary (or Madonna) summit (Cima della Madonna, Pointe de la Madone or Pointe de la Vierge – 4058 m) comprises rock climbing with considerable exposure, with difficulties of grade I UIAA, while access to the proper main summit (4061 m) requires 15 minutes of climbing up to grade II UIAA. Gran Paradiso is popular with novice alpinists and many of these only climb to the Madonna summit.

In addition to the main summit routes, Gran Paradiso possesses a steep snow/ice North West Face, which can be climbed at AD+ from the Refuge Frédéric Chabod.

Nature conservation
Gran Paradiso is located in the Gran Paradiso National Park, an Italian national park named after the mountain. On the French side of the border, the park is continued by the Vanoise National Park.

Maps

 * Italian official cartography (Istituto Geografico Militare - IGM); on-line version: www.pcn.minambiente.it
 * I.G.C. (Istituto Geografico Centrale) - Carta dei sentieri e dei rifugi 1:50.000 scale n.3 Parco Nazionale del Gran Paradiso and 1:25.000 n.101 Gran Paradiso, La Grivola, Cogne

title: Gran Paradiso


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User:Sirscuba

Hi Sirscuba, welcome to Wikipedia! Please read Wikipedia is not a dictionary before you add any more short articles about SCUBA diving. Thanks, Chadloder 17:49, Aug 7, 2003 (UTC)

title: User:Sirscuba


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USS Cassin

Two ships of the United States Navy have been named Cassin, in honor of Captain Stephen Cassin.

 * USS Cassin (DD-43), was a destroyer commissioned in 1913.
 * USS Cassin (DD-372), was a destroyer that served during World War II.

title: USS Cassin


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CYP

CYP may refer to:

 * CYP, IATA airport code for Calbayog Airport in the Philippines
 * CYP, national railway code for Crystal Palace railway station in London, UK
 * CYP, ISO 3166-1 alpha-3 country code for Cyprus
 * CYP, ISO 4217 code for the Cypriot pound
 * Cyclophilin
 * Cytochrome P450, isoenzyme
 * Cypripedium, orchid genus

title: CYP


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Technical diving

Technical diving (also referred to as tec diving or tech diving) is scuba diving that exceeds the agency-specified limits of recreational diving for non-professional purposes. Technical diving may expose the diver to hazards beyond those normally associated with recreational diving, and to a greater risk of serious injury or death. Risk may be reduced via appropriate skills, knowledge, and experience. Risk can also be managed by using suitable equipment and procedures. The skills may be developed through specialized training and experience. The equipment involves breathing gases other than air or standard nitrox mixtures, and multiple gas sources.

The popularisation of the term technical diving has been credited to Michael Menduno, who was editor of the (now defunct) diving magazine aquaCorps Journal, but the concept and term, technical diving, go back at least as far as 1977, and divers have been engaging in what is now commonly referred to as technical diving for decades.

Origin
The popular use of the term technical diving can be traced back to the cover story of the first issue of aquaCorps magazine (1990–1996), in early 1990, titled Call it "High-Tech" Diving by Bill Hamilton, describing the current state of recreational diving beyond the generally accepted limits, such as deep, decompression and mixed gas diving. By mid-1991, the magazine was using the term technical diving, as an analogy to the established term technical (rock) climbing.

More recently, recognizing that the term was already in use by the Royal Navy for rebreather diving, Hamilton redefined technical diving as diving with more than one breathing gas or with a rebreather. Richard Pyle (1999) defined a technical diver as "anyone who routinely conducts dives with staged stops during an ascent as suggested by a given decompression algorithm".

The term technical diving was also used in the US as far back as 1977 by the California Advisory Committee on Scientific and Technical Diving (CACSTD), to distinguish more complex modes of recreational diving from scientific diving for regulatory purposes. In the US the Occupational Safety and Health Administration categorises diving which is not occupational as recreational diving for purposes of exemption from regulation. This is also the case in some other countries, including South Africa.

Technical diving emerged between the mid-1980s and the mid-to-late-1990s, and much of the history of its development was recorded in aquaCorps, started by Michael Menduno to provide a forum for these aspects of diving that most recreational diving magazines of the time refused to cover.

At the time, amateur scuba divers were exploring the physiological limits of diving using air. Technical divers looked for ways to extend the limits of air dives, and for ways to extend breathing gas supplies as they went deeper and stayed down longer. The military and commercial diving communities had large budgets, extensive infrastructure, and controlled diving operations, but the amateur diving community had a more trial-and-error approach to the use of mixed gas and rebreathers. Consequently, a relatively large number of fatal incidents occurred during the early years, before a reasonably reliable set of operating procedures and standards began to emerge, making the movement somewhat controversial, both within the mainstream diving establishment and between sectors of the technical diving community.

While the motivation to extend the depth and duration range by military and commercial divers was mainly driven by operational needs to get the job done, the motivation to exceed recreational diving depths and endurance ranges was more driven by the urge to explore otherwise inaccessible places, which could not at the time be reached by any other means.

There are places that no one has been to since the dawn of time. We can’t see what’s there. We can see what’s on the dark side of the moon or what’s on Mars, but you can’t see what’s in the back of a cave unless you go there.

Sheck Exley, Exley on Mix, aquaCorps #4, Jan 1992

The urge to go where no one has gone before has always been a driving force for explorers, and the 1980s was a time of intense exploration by the cave-diving community, some of whom were doing relatively long air dives in the 60–125 m depth range, and doing decompression on oxygen. The details of many of these dives were not disclosed by the divers as these dives were considered experimental and dangerous. The divers who conducted these dives did not consider them suitable for the ordinary person, but necessary to extend the frontiers of exploration, and there were no consensus guidelines for scuba diving beyond 40 m.

Definition

There is some professional disagreement as to what exactly technical diving encompasses. Nitrox diving and rebreather diving were originally considered technical, but this is no longer universally the case as several certification agencies now offer Recreational Nitrox and recreational rebreather training and certification. Some training agencies classify penetration diving in wrecks and caves as technical diving. Even those who agree on the broad definitions of technical diving may disagree on the precise boundaries between technical and recreational diving.
 * IANTD proposes a description: "Technical diving is a range of knowledge, skills and suitable equipment, which when combined correctly, allow recreational divers to increase their safety while underwater. This information (sic) may be employed in either shallow or deep water, may be used to safely extend the divers submerged duration well into the realms of extended decompressions and is often used as a tool for exploration." in their Exploration and Mixed Gas Diving Encyclopedia
 * NAUI's definition of technical diving states: "Technical diving is a form of scuba diving that exceeds the typical recreational limits imposed on depth and immersion time (bottom time). Tec diving involves accelerated decompression and/or the use of variable gas mixtures during a dive."
 * NOAA defines technical diving as "all diving methods that exceed the limits imposed on depth and/or immersion time for recreational scuba diving. Technical diving often involves the use of special gas mixtures (other than compressed air) for breathing. The type of gas mixture used is determined either by the maximum depth planned for the dive, or by the length of time that the diver intends to spend underwater. While the recommended maximum depth for conventional scuba diving is 130 ft, technical divers may work in the range of 170 ft to 350 ft, sometimes even deeper. Technical diving almost always requires one or more mandatory decompression 'stops' upon ascent, during which the diver may change breathing gas mixes at least once." NOAA does not address issues relating to overhead environments or specify the recreational diving limits in its definition, and the use of single mixture nitrox is well established in mainstream recreational diving.
 * PADI defines technical diving as "diving other than conventional commercial or recreational diving that takes divers beyond recreational diving limits (130 ft). It is further defined as an activity that includes one or more of the following: diving beyond 40 meters/130 feet, required stage decompression, diving in an overhead environment beyond 130 linear feet from the surface, accelerated stage decompression and/or the use of multiple gas mixtures in a single dive."
 * TDI defines a technical dive as any dive involving decompression, additional cylinders, alternative breathing gases, rebreathers, or overhead environments such as wrecks, caves or mines. This definition does not make a strong distinction between “recreational” and “technical” as both styles of diving are recreational and require similar equipment.
 * The government of Queensland, Australia defines recreational technical diving as recreational diving using nitrox or other mixed gas, or any diving requiring decompression.
 * BSAC varies from many agencies in allowing some staged decompression within recreational diving. Its definition of technical diving is diving that involves specialist equipment such as Closed Circuit Rebreathers (CCR), using multiple gas mixes on open circuit or that uses helium-based gas mixtures termed Mixed Gas.

The European diving agencies tend to draw the line between recreational and technical diving at 50 m and many, as noted for BSAC above, teach staged decompression diving as an integral part of recreational training, rather than as a fundamental change of scope. The Bühlmann tables used by the Sub-Aqua Association and other European agencies make staged decompression dives available, and the SAA teaches modest staged decompression as part of its advanced training programme.

Scope
The following table gives an overview of the activities that various agencies suggest to differentiate between technical and recreational diving:

Hazards and risk
One of the perceived differences between technical and other forms of recreational diving is the associated hazards, of which there are more associated with technical diving, and risk, which is often, but not always greater in technical diving. Hazards are the circumstances that may cause harm, and risk is the likelihood of the harm actually occurring. The hazards are partly due to the extended scope of technical diving, and partly associated with the equipment used. In some cases, the equipment used presents a secondary risk while mitigating a primary risk, such as the complexity of gas management needed to reduce the risk of a fatal gas supply failure, or the use of gases potentially unbreathable for some parts of a dive profile to reduce the risk of harm caused by oxygen toxicity, nitrogen narcosis or decompression sickness for the whole operation. Reduction of secondary risks may also affect equipment choice, but is largely skill-based. Training of technical divers includes procedures that are known from experience to be effective in handling the most common contingencies. Divers proficient in these emergency drills are less likely to be overwhelmed by the circumstances when things do not go according to plan, and are less likely to panic.

Depth
Technical dives may be defined as being dives deeper than about 130 ft or dives in an overhead environment with no direct access to the surface or natural light. Such environments may include fresh and saltwater caves and the interiors of shipwrecks. In many cases, technical dives also include planned decompression carried out over a number of stages during a controlled ascent to the surface at the end of the dive. The depth-based definition is based on risk caused by the progressive impairment of mental competence with the increasing partial pressure of respired nitrogen. Breathing air under pressure causes nitrogen narcosis that usually starts to become a problem at depths of 100 ft or greater, but this differs between divers. Increased depth also increases the partial pressure of oxygen and so increases the risk of oxygen toxicity. Technical diving often includes the use of breathing mixtures other than air to reduce these risks, and the additional complexity of managing a variety of breathing mixtures introduces other risks and is managed by equipment configuration and procedural training. To reduce nitrogen narcosis, it is common to use trimix which uses helium to replace some of the nitrogen in the diver's breathing mixture, or heliox, in which there is no nitrogen.

Inability to ascend directly
Technical dives may alternatively be defined as dives where the diver cannot safely ascend directly to the surface either due to a mandatory decompression stop or a physical ceiling. This form of diving implies a much larger reliance on the redundancy of critical equipment and procedural training since the diver must stay underwater until it is safe to ascend or the diver has successfully exited the overhead environment.

Decompression stops

A diver at the end of a long or deep dive may need to do decompression stops to avoid decompression sickness, also known as "the bends". Metabolically inert gases in the diver's breathing gas, such as nitrogen and helium, are absorbed into body tissues when breathed under high pressure, mainly during the deep phase of the dive. These dissolved gases must be released slowly from the body tissues by controlling the ascent rate to restrict the formation and growth of bubbles. This is usually done by pausing or "doing stops" at various depths during the ascent to the surface. Most technical divers breathe oxygen enriched breathing gas mixtures such as nitrox and pure oxygen during long-duration decompression, as this increases the rate of inert gas elimination. Elimination of inert gases continues during the surface intervals (time spent on the surface between dives), which must be considered when planning subsequent dives. A decompression obligation is also referred to as a "soft", or "physiological" ceiling.

Physical ceiling
These types of physical overhead, or "hard" or "environmental" ceiling can prevent the diver from surfacing directly: In all three of these situations, a guide line or lifeline from the exit to the diver is the standard method of reducing the risk of being unable to find the way out. A lifeline fixed to the diver is more reliable as it is not easy to lose, and is often used when diving under ice, where the line is unlikely to snag and the distance is reasonably short, and can be tended by a person at the surface. Static guidelines are more suitable when a lifeline is likely to snag on the environment or on other divers in the group, and may be left in situ to be used for other dives, or recovered on the way out by winding back onto the reel. Guidelines may be very much longer than lifelines, and may be branched and marked. They are used as standard practice for cave diving and wreck penetration.
 * Cave diving – diving into a cave system.
 * Ice diving – diving under ice.
 * Wreck diving – diving inside a shipwreck.

Extremely limited visibility
Technical dives in waters where the diver's vision is severely impeded by low-visibility conditions, caused by turbidity or silt out and low light conditions due to depth or enclosure, require greater competence. The combination of low visibility and strong current can make dives in these conditions extremely hazardous, particularly in an overhead environment, and greater skill and reliable and familiar equipment are needed to manage this risk. Limited visibility diving can cause disorientation, potentially leading to loss of sense of direction, loss of effective buoyancy control, etc. Divers in extremely limited visibility situations depend on their instruments such as dive lights, pressure gauges, compass, depth gauge, bottom timer, dive computer, etc., and guidelines for orientation and information. Training for cave and wreck diving includes techniques for managing extreme low visibility, as finding the way out of an overhead environment before running out of gas is a safety-critical skill.

Equipment

Technical divers may use diving equipment other than the usual single cylinder open circuit scuba equipment used by recreational divers. Typically, technical dives take longer than average recreational scuba dives. Because a decompression obligation prevents a diver in difficulty from surfacing immediately, there is a need for redundancy of breathing equipment. Technical divers usually carry at least two independent breathing gas sources, each with its own gas delivery system. In the event of a failure of one set, the second set is available as a back-up system. The backup system should allow the diver to safely return to the surface from any point of the planned dive, but may involve the intervention of other divers in the team. Stage cylinders may be dropped along the guideline for later use during the exit or for another dive.

Equipment configuration
The usual configurations used for increased primary gas supply are manifolded or independent twin back mounted cylinders, multiple side mounted cylinders, or rebreathers. Bailout and decompression gas may be included in these arrangements, or carried separately as side-mounted stage and decompression cylinders. Cylinders may carry a variety of gases depending on when and where they will be used, and as some may not support life if used at the wrong depth, they are marked for positive identification of the contents. Managing the larger number of cylinders is an additional task loading on the diver. Cylinders are usually labeled with the gas mixture and will also be marked with the maximum operating depth and if applicable, minimum operating depth.

Gas mixes
Technical diving can be done using air as a breathing gas, but other breathing gas mixtures are commonly used to manage specific problems. Some additional knowledge is required to understand the effects of these gases on the body during a dive and additional skills are needed to safely manage their use.

Deep air/extended range diving
One of the more divisive subjects in technical diving concerns using compressed air as a breathing gas on dives below 130 ft. Some training agencies still promote and teach courses using air up to depths of 60m. These include TDI, IANTD and DSAT/PADI. Others, including NAUI Tec, GUE, ISE and UTD consider that diving deeper than 100 –, depending upon agency, on air is unacceptably risky. They promote the use of mixtures containing helium to limit the apparent narcotic depth to their agency specified limit should be used for dives beyond a certain limit. Even though TDI and IANTD teach courses using air up to depths of 60m, they also offer courses include "helitrox" "recreational trimix" and "advance recreational trimix" that also use mixtures containing helium to mitigate narcotic concerns when the diving depth is limited to 30-45m.

Such courses used to be referred to as "deep air" courses, but are now commonly called "extended range" courses. The 130 ft limit entered the recreation and technical communities in the USA from the military diving community where it was the depth at which the US Navy recommended shifting from scuba to surface-supplied air. The scientific diving community has never specified a 130-foot limit in its protocols and has never experienced any accidents or injuries during air dives between 130 feet and the deepest air dives that the scientific diving community permits, 190 feet, where the U.S. Navy Standard Air Tables shifts to the Exceptional Exposure Tables. In Europe, some countries set the recreational diving limit at 50 m, and that corresponds with the limit also imposed in some professional fields, such as police divers in the UK. The major French agencies all teach diving on air to 60 m as part of their standard recreational certifications.

Deep air proponents base the depth limit of air diving upon the risk of oxygen toxicity. Accordingly, they view the limit as being the depth at which partial pressure of oxygen reaches 1.4 ATA, which occurs at about 186 ft. Both sides of the community tend to present self-supporting data. Divers trained and experienced in deep air diving report fewer problems with narcosis than those trained and experienced in mixed gas diving trimix/heliox, though scientific evidence does not show that a diver can train to overcome any measure of narcosis at a given depth or become tolerant of it.

The Divers Alert Network does not endorse or reject deep air diving but does note the additional risks involved.

Mixtures to reduce decompression time
Nitrox is a popular diving gas mix, that reduces the maximum allowable depth as compared to air. Nitrox also allows greater bottom time and shorter surface intervals by reducing the buildup of nitrogen in the diver's tissues. This is accomplished by increasing the percentage of oxygen in the breathing gas. The depth limit of a nitrox mixture is governed by the partial pressure of oxygen, which is generally limited to 1.4 to 1.6 bar depending on the activity of the diver and duration of exposure.

Nitrox mixtures up to 100% oxygen are also used for accelerated decompression.

Mixtures to reduce nitrogen narcosis
Increased pressure due to depth causes nitrogen to become narcotic, resulting in a reduced ability to react or think clearly. By adding helium to the breathing mix, these effects can be reduced, as helium does not have the same narcotic properties at depth. Helitrox/triox proponents argue that the defining risk for air and nitrox diving depth should be nitrogen narcosis, and suggest that when the partial pressure of nitrogen reaches approximately 4.0 ATA, which occurs at about 130 ft for air, helium is necessary to limit the effects of the narcosis.

Mixtures to reduce oxygen toxicity
Technical dives may also be characterised by the use of hypoxic breathing gas mixtures, including hypoxic trimix, heliox, and heliair. A diver breathing normal air (with 21% oxygen) will be exposed to increased risk of central nervous system oxygen toxicity at depths greater than about 180 ft The first sign of oxygen toxicity is usually a convulsion without warning which usually results in death when the demand valve mouthpiece falls out and the victim drowns. Sometimes the diver may get warning symptoms before the convulsion. These can include visual and auditory hallucinations, nausea, twitching (especially in the face and hands), irritability and mood swings, and dizziness.

These gas mixes can also lower the level of oxygen in the mix to reduce the danger of oxygen toxicity. Once the oxygen is reduced below about 18% the mix is known as a hypoxic mix as it does not contain enough oxygen to be used safely at the surface.

Safety
Technical diving encompasses multiple aspects of diving, that typically share a lack of direct access to the surface, which may be caused by physical constraints, like an overhead environment, or physiological, like decompression obligation. In case of emergency, therefore, the diver or diving team must be able to troubleshoot and solve the problem underwater. This requires planning, situational awareness, and redundancy in critical equipment, and is facilitated by skill and experience in appropriate procedures for managing reasonably foreseeable contingencies.

Some rebreather diving safety issues can be addressed by training, others may require a change in technical diver culture. A major safety issue is that many divers become complacent as they become more familiar with the equipment, and begin to neglect predive checklists while assembling and preparing the equipment for use - procedures that are officially part of all rebreather training programs. There can also be a tendency to neglect post-dive maintenance, and some divers will dive knowing that there are functional problems with the unit, because they know that there is generally redundancy designed into the system. This redundancy is intended to allow a safe termination of the dive if it occurs underwater, by eliminating a critical failure point. Diving with a unit that already has a malfunction, means that there is a single critical point of failure in that unit, which could cause a life-threatening emergency if another item in the critical path were to fail. The risk may increase by orders of magnitude.

Accident modes
Several factors have been identified as predispositions to accidents in technical diving. The techniques and equipment are complex, which increases the risk of errors or omissions - the task loading for a closed circuit rebreather diver during critical phases of a dive is greater than for open circuit scuba equipment, The circumstances of technical diving generally mean that errors or omissions are likely to have more serious consequences than in normal recreational diving, and there is a tendency towards competitiveness and risk-taking among many technical divers which appears to have contributed to some well-publicized accidents.

Some errors and failures that have repeatedly been implicated in technical diving accidents include:
 * Incorrect gas switches in open circuit diving; The gas could be hypoxic, with a risk of blackout, hyperoxic, with a risk of oxygen toxicity seizure, or have an excessively high partial pressure of nitrogen, with a risk of nitrogen narcosis.
 * Having an incorrect gas in a cylinder resulting in hypoxia, hyperoxia, nitrogen narcosis or inadequate decompression, usually a consequence of failure to analyse all the mixes;
 * Incorrect gas consumption calculations and failure to monitor use and change plans during the dive, causing running out of gas before the end of the dive;
 * Losing staged decompression gas which was cached to be picked up later;
 * The development of an insufficient or excessive oxygen partial pressure in the loop of closed or semi-closed circuit rebreathers;
 * High CO2 levels in the breathing loop of rebreathers due to scrubber breakthrough;
 * Flooding of the rebreather loop rendering it unusable.
 * Failure to control depth.

Failure to control depth is critical during decompression, where the inability to stay at the correct depth due to excessive buoyancy is associated with a high risk of decompression sickness and a raised risk of barotrauma of ascent. There are several ways that excessive buoyancy can be caused, some of which can be managed by the diver if prompt and correct action is taken, and others that cannot be corrected. This problem may be caused by poor planning, in that the diver may underestimate the weight loss of using up the breathing gas in all the cylinders, by losing ballast weights during the dive, or by inflation problems with buoyancy compensator or drysuit, or both.

Insufficient ballast weight to allow neutral buoyancy at the shallowest decompression stop with nearly empty cylinders is an example of a buoyancy problem that can generally not be corrected by the diver. If an empty cylinder is positively buoyant, the diver may jettison it and allow it to float away, but if the empty cylinders are negatively buoyant, jettisoning them will exacerbate the problem, making the diver even more buoyant. Drysuit and buoyancy compensator inflation can cause runaway ascent, which can usually be managed if corrected immediately. If the initial problem is caused by loss of ballast weights or a reel jam when deploying an inflatable decompression buoy, and the reel is clipped on, the diver may not be able to manage several simultaneously accelerating buoyancy malfunctions. Dual bladder buoyancy compensators can contain air inadvertently added to the backup bladder, which the diver does not release as it is not supposed to be there in the first place. All of these failures can be either avoided altogether or the risk minimized by configuration choices, procedural methods, and correct response to the initial problem.

Failure to control depth due to insufficient buoyancy can also lead to scuba accidents. It is less of a problem with surface-supplied diving as the depth that the diver can sink to is limited by the umbilical length, and a sudden or rapid descent can often be quickly stopped by the tender. In early diving using copper helmets and a limited flow air supply, a sudden rapid descent could lead to severe helmet squeeze, but this is prevented by demand-supplied gas, and neck dams on later helmets, which allow water to flood the helmet until the gas supply catches up with the compression. Surface supply ensures that the gas supply will not run out suddenly due to high demand, which can deplete scuba supply to the extent that there may not be enough left to surface according to plan. Any sudden increase in depth can also cause barotrauma of the ears and sinuses if the diver cannot equalize fast enough.

Accident statistics
There is very little reliable data describing the demographics, activities and accidents of the technical diving population. Conclusions about accident rates must be considered tentative. The 2003 DAN report on decompression illness and dive fatalities indicates that 9.8% of all cases of decompression illness and 20% of diving fatalities in the USA happened to technical divers. It is not known how many technical dives this was spread over, but it was considered likely that technical divers are at greater risk.

The techniques and associated equipment that have been developed to overcome the limitations of conventional single-cylinder, open-circuit scuba diving are necessarily more complex and subject to error, and technical dives are often done in more dangerous environments, so the consequences of an error or malfunction are greater. Although the skill levels and training of technical divers are generally significantly higher than those of recreational divers, there are indications that technical divers, in general, are at higher risk, and that closed circuit rebreather diving may be particularly dangerous.

Operations
Relatively complex technical diving operations may be planned and run like an expedition, or professional diving operation, with surface and in-water support personnel providing direct assistance or on stand-by to assist the expedition divers. Surface support might include surface stand-by divers, boat crew, porters, emergency medical personnel, and gas blenders. In-water support may provide supplementary breathing gas, monitor divers during long decompression stops, and provide communications services between the surface team and the expedition divers. In some cases the risk assessment may persuade the dive team to use similar equipment to that used in professional diving, such as ROV monitoring or the use of a stage or wet bell for the ascent and descent, and having a decompression chamber available at the surface. In an emergency, the support team would provide rescue and if necessary search and recovery assistance.

Training

Technical diving requires specialized equipment and training. There are many technical training organizations: see the Technical Diving section in the list of diver certification organizations. Technical Diving International (TDI), Global Underwater Explorers (GUE), Professional Scuba Association International (PSAI), International Association of Nitrox and Technical Divers (IANTD) and National Association of Underwater Instructors (NAUI) were popular. Professional Technical and Recreational Diving (ProTec) joined in 1997. Recent entries into the market include Split-Face Diving (UTD), InnerSpace Explorers (ISE) and Diving Science and Technology (DSAT), the technical arm of Professional Association of Diving Instructors (PADI). The Scuba Schools International (SSI) Technical Diving Program (TechXR – Technical eXtended Range) was launched in 2005.

British Sub-Aqua Club (BSAC) training has always had a technical element to its higher qualifications, however, it has recently begun to introduce more technical level Skill Development Courses into all its training schemes by introducing technical awareness into its lowest level qualification of Ocean Diver, for example, and nitrox training will become mandatory. It has also recently introduced trimix qualifications and continues to develop closed-circuit training.

Certification
Technical diving certification is issued by several recreational diver training agencies, under a variety of names, often with considerable overlap or in some cases split into depth ranges. The certification titles vary between agencies but can be categorized as:
 * , a scuba diver certified as competent to dive in open water using open circuit nitrogen-based gas mixtures that are safe to breathe at atmospheric pressure, and decompress using approved schedules on gases carried by the diver, usually including oxygen or oxygen-rich nitrox
 * , a scuba diver certified as competent to dive in open water using open circuit trimix gases which are safe to breathe at atmospheric pressure, and decompress using approved schedules, on gases carried by the diver, usually including oxygen or oxygen-rich nitrox.
 * , or, a scuba diver certified as competent to dive in open water using open circuit trimix gases which are not safe to breathe at atmospheric pressure, to use a travel gas to descend through the depth range in which the bottom gas is unsafe, and decompress using approved schedules, on gases carried by the diver, usually including oxygen or oxygen-rich nitrox.
 * Rebreather certification for various types of diving rebreather. Training in the use of rebreathers has two components: Generic training for the class of rebreather, including the theory of operation, general procedures as well as specific training for the model of rebreather; which covers the details of preparation, testing, user maintenance, and troubleshooting, and those details of normal operating and emergency procedures which are specific to the model of rebreather. Crossover training from one model to another generally only requires the second aspect if the equipment is similar in design and operation.
 * Nitrox semi-closed circuit rebreather diver,
 * Nitrox closed circuit rebreather diver,
 * Mixed gas rebreather diver. This may distinguish between normoxic and hypoxic depth ranges.
 * Cave diver, of various grades,
 * Wreck penetration diver, of various grades,

title: Technical diving


text:
Deep diving

Deep diving is underwater diving to a depth beyond the norm accepted by the associated community. In some cases this is a prescribed limit established by an authority, while in others it is associated with a level of certification or training, and it may vary depending on whether the diving is recreational, technical or commercial. Nitrogen narcosis becomes a hazard below 30 m and hypoxic breathing gas is required below 60 m to lessen the risk of oxygen toxicity.

For some recreational diving agencies, "Deep diving", or "Deep diver" may be a certification awarded to divers that have been trained to dive to a specified depth range, generally deeper than 30 m. However, the Professional Association of Diving Instructors (PADI) defines anything from 18 to 30 m as a "deep dive" in the context of recreational diving (other diving organisations vary), and considers deep diving a form of technical diving. In technical diving, a depth below about 60 m where hypoxic breathing gas becomes necessary to avoid oxygen toxicity may be considered a deep dive. In professional diving, a depth that requires special equipment, procedures, or advanced training may be considered a deep dive.

Deep diving can mean something else in the commercial diving field. For instance early experiments carried out by COMEX using heliox and trimix attained far greater depths than any recreational technical diving. One example being its "Janus 4" open-sea dive to 501 m in 1977.

The open-sea diving depth record was achieved in 1988 by a team of COMEX and French Navy divers who performed pipeline connection exercises at a depth of 534 m in the Mediterranean Sea as part of the "Hydra 8" programme employing heliox and hydrox. The latter avoids the high-pressure nervous syndrome (HPNS) caused by helium and eases breathing due to its lower density. These divers needed to breathe special gas mixtures because they were exposed to very high ambient pressure (more than 54 times atmospheric pressure).

An atmospheric diving suit (ADS) allows very deep dives of up to 2300 ft. These suits are capable of withstanding the pressure at great depth permitting the diver to remain at normal atmospheric pressure. This eliminates the problems associated with breathing pressurised gases. In 2006 Chief Navy Diver Daniel Jackson set a record of 2000 ft in an ADS.

On 20 November 1992 COMEX's "Hydra 10" experiment simulated a dive in an onshore hyperbaric chamber with hydreliox. Théo Mavrostomos spent two hours at a simulated depth of 701 m.

Depth ranges in underwater diving
Assumed is the surface of the waterbody to be at or near sea level and underlies atmospheric pressure.

Not included are the differing ranges of freediving – without breathing during a dive.

Particular problems associated with deep dives
Deep diving has more hazards and greater risk than basic open-water diving. Nitrogen narcosis, the "narks" or "rapture of the deep", starts with feelings of euphoria and over-confidence but then leads to numbness and memory impairment similar to alcohol intoxication. Decompression sickness, or the "bends", can happen if a diver ascends too rapidly, when excess inert gas leaves solution in the blood and tissues and forms bubbles. These bubbles produce mechanical and biochemical effects that lead to the condition. The onset of symptoms depends on the severity of the tissue gas loading and may develop during ascent in severe cases, but is frequently delayed until after reaching the surface. Bone degeneration (dysbaric osteonecrosis) is caused by the bubbles forming inside the bones; most commonly the upper arm and the thighs. Deep diving involves a much greater danger of all of these, and presents the additional risk of oxygen toxicity, which may lead to convulsions underwater. Very deep diving using a helium-oxygen mixture (heliox) or a hydrogen-helium-oxygen mixture (hydreliox) carries the risk of high-pressure nervous syndrome and hydrogen narcosis. Coping with the physical and physiological stresses of deep diving requires good physical conditioning.

Using open-circuit scuba equipment, consumption of breathing gas is proportional to ambient pressure – so at 50 m, where the pressure is 6 bar, a diver breathes six times as much as on the surface (1 bar). Heavy physical exertion makes the diver breathe even more gas, and gas becomes denser requiring increased effort to breathe with depth, leading to increased risk of hypercapnia – an excess of carbon dioxide in the blood. The need to do decompression stops increases with depth. A diver at 6 m may be able to dive for many hours without needing to do decompression stops. At depths greater than 40 m, a diver may have only a few minutes at the deepest part of the dive before decompression stops are needed. In the event of an emergency, the diver cannot make an immediate ascent to the surface without risking decompression sickness. All of these considerations result in the amount of breathing gas required for deep diving being much greater than for shallow open water diving. The diver needs a disciplined approach to planning and conducting dives to minimise these additional risks.

Many of these problems are avoided by the use of surface supplied breathing gas, closed diving bells, and saturation diving, at the cost of logistical complexity, reduced maneuverability of the diver, and greater expense.

Dealing with depth

Both equipment and procedures can be adapted to deal with the problems of greater depth. Usually the two are combined, as the procedures must be adapted to suit the equipment, and in some cases the equipment is needed to facilitate the procedures.

Equipment adaptations for deeper diving
The equipment used for deep diving depends on both the depth and the type of diving. Scuba is limited to equipment that can be carried by the diver or is easily deployed by the dive team, while surface-supplied diving equipment can be more extensive, and much of it stays above the water where it is operated by the diving support team.
 * Scuba divers carry larger volumes of breathing gas to compensate for the increased gas consumption and decompression stops.
 * Rebreathers, though more complex, manage gas much more efficiently than open-circuit scuba.
 * Use of helium-based breathing gases such as trimix reduces nitrogen narcosis and reduces the toxic effects of oxygen at depth.
 * A diving shot, a decompression trapeze, or a decompression buoy can help divers control their ascent and return to the surface at a position that can be monitored by their surface support team at the end of a dive.
 * Decompression can be accelerated by using specially blended breathing gas mixtures containing lower proportions of inert gas.
 * Surface supply of breathing gases reduces the risk of running out of gas.
 * In-water decompression can be minimized by using dry bells and decompression chambers.
 * Hot-water suits can prevent hypothermia due to the high heat loss when using helium-based breathing gases.
 * Diving bells and submersibles expose the diver to the direct underwater environment for less time, and provide a relatively safe shelter that does not require decompression, with a dry environment where the diver can rest, take refreshment, and if necessary, receive first aid in an emergency.
 * Breathing gas s reduce the cost of using helium-based breathing gases, by recovering and recycling exhaled surface supplied gas, analogous to rebreathers for scuba diving.
 * The most radical equipment adaptation for deep diving is to isolate the diver from the direct pressure of the environment, using armoured atmospheric diving suits that allow diving to depths beyond those currently possible at ambient pressure. These rigid, articulated exoskeleton suits are sealed against water and withstand external pressure while providing life support to the diver for several hours at an internal pressure of approximately normal surface atmospheric pressure. This avoids the problems of inert gas narcosis, decompression sickness, barotrauma, oxygen toxicity, high work of breathing, compression arthralgia, high-pressure nervous syndrome and hypothermia, but at the cost of reduced mobility and dexterity, logistical problems due to the bulk and mass of the suits, and high equipment costs.

Procedural adaptations for deeper diving
Procedural adaptations for deep diving can be classified as those procedures for operating specialized equipment, and those that apply directly to the problems caused by exposure to high ambient pressures.
 * The most important procedure for dealing with physiological problems of breathing at high ambient pressures associated with deep diving is decompression. This is necessary to prevent inert gas bubble formation in the body tissues of the diver, which can cause severe injury. Decompression procedures have been derived for a large range of pressure exposures, using a large range of gas mixtures. These basically entail a slow and controlled reduction in pressure during ascent by using a restricted ascent rate and decompression stops, so that the inert gases dissolved in the tissues of the diver can be eliminated harmlessly during normal respiration.
 * Gas management procedures are necessary to ensure that the diver has access to suitable and sufficient breathing gas at all times during the dive, both for the planned dive profile and for any reasonably foreseeable contingency. Scuba gas management is logistically more complex than surface supply, as the diver must either carry all the gas, must follow a route where previously arranged gas supply depots have been set up (stage cylinders). or must rely on a team of support divers who will provide additional gas at pre-arranged signals or points on the planned dive. On very deep scuba dives or on occasions where long decompression times are planned, it is a common practice for support divers to meet the primary team at decompression stops to check if they need assistance, and these support divers will often carry extra gas supplies in case of need.
 * Rebreather diving can reduce the bulk of the gas supplies for long and deep scuba dives, at the cost of more complex equipment with more potential failure modes, requiring more demanding procedures and higher procedural task loading.
 * Surface supplied diving distributes the task loading between the divers and the support team, who remain in the relative safety and comfort of the surface control position. Gas supplies are limited only by what is available at the control position, and the diver only needs to carry sufficient bailout capacity to reach the nearest place of safety, which may be a diving bell or lockout submersible.
 * Saturation diving is a procedure used to reduce the high-risk decompression a diver is exposed to during a long series of deep underwater exposures. By keeping the diver under high pressure for the whole job, and only decompressing at the end of several days to weeks of underwater work, a single decompression can be done at a slower rate without adding much overall time to the job. During the saturation period, the diver lives in a pressurized environment at the surface, and is transported under pressure to the underwater work site in a closed diving bell.

Mixed gas
Amongst technical divers, there are divers who participate in ultra-deep diving on scuba below 200 m. This practice requires high levels of training, experience, discipline, fitness and surface support. Only twenty-six people are known to have ever dived to at least 240 m on self-contained breathing apparatus recreationally. The "Holy Grail" of deep scuba diving was the 300 m mark, first achieved by John Bennett in 2001, and has only been achieved five times since.

The difficulties involved in ultra-deep diving are numerous. Although commercial and military divers often operate at those depths, or even deeper, they are surface supplied. All of the complexities of ultra-deep diving are magnified by the requirement of the diver to carry (or provide for) their own gas underwater. These lead to rapid descents and "bounce dives". This has led to extremely high mortality rates amongst those who practise ultra-deep diving. Notable ultra-deep diving fatalities include Sheck Exley, John Bennett, Dave Shaw and Guy Garman. Mark Ellyatt, Don Shirley and Pascal Bernabé were involved in serious incidents and were fortunate to survive their dives. Despite the extremely high mortality rate, the Guinness World Records continues to maintain a record for scuba diving (although the record for deep diving with compressed air has not been updated since 1999, given the high accident rate). Amongst those who do survive significant health issues are reported. Mark Ellyatt is reported to have suffered permanent lung damage; Pascal Bernabé (who was injured on his dive when a light on his mask imploded ) and Nuno Gomes reported short to medium term hearing loss.

Serious issues that confront divers engaging in ultra-deep diving on self-contained breathing apparatus include:
 * Compression arthralgia: Deep aching pain in the knees, shoulders, fingers, back, hips, neck, and ribs caused by exposure to high ambient pressure at a relatively high rate of descent (i.e., in "bounce dives").
 * High-pressure nervous syndrome (HPNS): HPNS, brought on by breathing helium under extreme pressure causes tremors, myoclonic jerking, somnolence, EEG changes, visual disturbance, nausea, dizziness, and decreased mental performance. Symptoms of HPNS are exacerbated by rapid compression, a feature common to ultra-deep "bounce" dives.
 * Isobaric counterdiffusion (ICD): ICD is the diffusion of one inert gas into body tissues while another inert gas is diffusing out. It is a complication that can occur during decompression, and that can result in the formation or growth of bubbles without changes in the environmental pressure.
 * Decompression algorithm: There are no reliable decompression algorithms tested for such depths on the assumption of an immediate surfacing. Almost all decompression methodology for such depths is based upon saturation, and calculates ascent times in days rather than hours. Accordingly, ultra-deep dives are almost always a partly experimental basis.

In addition, "ordinary" risks like gas reserves, hypothermia, dehydration and oxygen toxicity are compounded by extreme depth and exposure. Much technical equipment is simply not designed for the necessarily greater stresses at depths, and reports of key equipment (including submersible pressure gauges) imploding are not uncommon.

Air
A severe risk in ultra-deep air diving is deep water blackout, or depth blackout, a loss of consciousness at depths below 50 m with no clear primary cause, associated with nitrogen narcosis, a neurological impairment with anaesthetic effects caused by high partial pressure of nitrogen dissolved in nerve tissue, and possibly acute oxygen toxicity. The term is not in widespread use at present, as where the actual cause of blackout is known, a more specific term is preferred. The depth at which deep water blackout occurs is extremely variable and unpredictable. Before the popular availability of trimix, attempts were made to set world record depths using air. The extreme risk of both narcosis and oxygen toxicity in the divers contributed to a high fatality rate in those attempting records. In his book, Deep Diving, Bret Gilliam chronicles the various fatal attempts to set records as well as the smaller number of successes. From the comparatively few who survived extremely deep air dives:

In deference to the high accident rate, the Guinness World Records have ceased to publish records for deep air dives, after Manion's dive.

Fatalities during depth record attempts

 * Maurice Fargues, a member of the GRS (Groupement de Recherches Sous-marines, Underwater Research Group headed by Jacques Cousteau), died in 1947 after losing consciousness at depth in an experiment to see how deep a scuba diver could go. He reached 120 m before failing to return line signals. He became the first diver to perish using an Aqua-Lung.
 * Hope Root died on 3 December 1953 off the coast of Miami Beach trying set a deep diving record of 410 ft with an Aqua-Lung; he passed 500 ft and was not seen again.
 * Archie Forfar and Ann Gunderson died on 11 December 1971 off the coast of Andros Island, while attempting to dive to 146 m, which would have been the world record at the time. Their third team member, Jim Lockwood, only survived due to his use of a safety weight that dropped when he lost consciousness at 122 m, causing him to start an uncontrolled ascent before being intercepted by a safety diver at a depth of around 300 ft. Sheck Exley, who was acting as another safety diver at 300 feet, inadvertently managed to set the depth record when he descended towards Forfar and Gunderson, who were both still alive at the 480-foot level, although completely incapacitated by narcosis. Exley was forced to give up his attempt at around 465 ft when the narcosis very nearly overcame him as well. The bodies of Forfar and Gunderson were never recovered.
 * Sheck Exley died in 1994 at 879 ft in an attempt to reach the bottom of Zacatón in a dive that would have extended his own world record (at the time) for deep diving.
 * Dave Shaw died in 2005 in an attempt at the deepest ever body recovery and deepest ever dive on a rebreather at 886 ft.
 * Brigitte Lenoir, planning to attempt the deepest dive ever made by a woman with a rebreather to 230 m, died on 14 May 2010 in Dahab while ascending from a training dive at 147 m.
 * Guy Garman died on 15 August 2015 in an unsuccessful attempt to dive to 1200 ft. The Virgin Island Police Department confirmed that Guy Garman's body was recovered on 18 August 2015.
 * Theodora Balabanova died at Toroneos Bay, Greece, in September 2017 attempting to break the women's deep dive record with 231 m. She did not complete the decompression stops and surfaced too early.
 * Wacław Lejko attempting 275 m in Lake Garda, died in September 2017. His body was recovered with a ROV at 230 m.
 * Adam Krzysztof Pawlik, attempting a 316 m dive in Lake Garda, died on 18 October 2018. His body was located at 284 m.
 * Sebastian Marczewski reached the target depth of 275 m in Lake Garda but his tanks became entangled in his ascent line at 150 m. He died on 6 July 2019.
 * Han Ting, having renewed his own 234 m deepest Asian cave dive record to 277 m in April 2023 in Tianchuang, planned to set a world record for deepest cave dive there, aiming at 300 m. He failed to return from a preparatory dive on 7 October 2023. His body was recovered by ROV on 25 October 2023.

title: Deep diving


text:
Herblock

Herbert Lawrence Block, commonly known as Herblock (October13, 1909 – October7, 2001), was an American editorial cartoonist and author best known for his commentaries on national domestic and foreign policy.

During the course of a career stretching into nine decades, he won three Pulitzer Prizes for editorial cartooning (1942, 1954, and 1979), shared a fourth Pulitzer Prize in 1973 for Public Service on Watergate, the Presidential Medal of Freedom (1994), the National Cartoonist Society Editorial Cartoon Award in 1957 and 1960, the Reuben Award in 1956, the Gold Key Award (the National Cartoonists Society Hall of Fame) in 1979, and numerous other honors.

Career
Block was the youngest of three boys born in Chicago to a Catholic mother, Theresa Lupe Block, and a father of Jewish descent, David Julian Block, a chemist and electrical engineer. His brother Rich became president of an industrial laundry and his brother Bill was a newspaper reporter for the Chicago Tribune and later for the Chicago Sun.

He began taking classes at the Art Institute of Chicago when he was eleven, and adopted the "Herblock" signature in high school.

After graduating in 1927, he attended Lake Forest College for almost two years.

Late in his second year there he was hired—after submitting some cartoons he had done in high school and college for the Evanston News-Index—to replace the Chicago Daily News' departing editorial cartoonist. He never returned to school.

Block moved to Cleveland in 1933 to become the staff cartoonist for Newspaper Enterprise Association, which distributed his cartoons nationally.

He won his first Pulitzer Prize in 1942, then spent two years in the Army doing cartoons and press releases. Upon discharge Block became chief editorial cartoonist for The Washington Post, where he worked until his death 55 years later.

Block's cartoons were syndicated to newspapers around the world by Creators Syndicate from 1987 until his death in 2001.

He never married, and, in the Post's employee index, his address was listed as simply "The Washington Post".

Cartoons
His first cartoon for the Chicago Daily News (April24, 1929) advocated conservation of America's forests. Herblock said that his family was conservative and that his father voted for Herbert Hoover in 1928. But with the onset of the Great Depression, he became a supporter of President Franklin D. Roosevelt and the New Deal. He pointed out the dangers of Soviet aggression, the growing Nazi menace, and opposed American isolationists.

While he criticized Stalin and other Communist figures, he also believed that the United States was overreacting to the danger of communism.

In the early 1950s, Senator Joseph McCarthy was a recurring target of Herblock's cartoons, one of which introduced the term McCarthyism. He won a second Pulitzer Prize in 1954. The Washington Post officially endorsed Eisenhower in the 1952 presidential election. Because Herblock supported Adlai Stevenson, the Post pulled his cartoons, but restored them after a week. He always insisted on total editorial independence, regardless of whether or not his cartoons agreed with the Post's stance on political issues. He focused most of his attacks on those public figures in power, often on Republican figures, but Democrats who displeased him were not immune from criticism. As an example—despite being an ardent admirer of Franklin Roosevelt—he found it necessary to attack the president's 1937 court-packing scheme.

During the 1950s, Herblock criticized Eisenhower mainly for insufficient action on civil rights and for not curbing the abuses of Senator McCarthy. In the following decade, he attacked the US war effort in Vietnam, causing President Johnson to drop his plans of awarding the cartoonist with a Presidential Medal of Freedom. The cartoonist would eventually be awarded this honor by Bill Clinton in 1994.

Some of Herblock's finest cartoons were those attacking the Nixon Administration during the Watergate Scandal, winning him his third Pulitzer Prize in 1979. Nixon canceled his subscription to the Post after Herblock drew him crawling out of an open sewer in 1954. He had once used the same motif for Senator McCarthy.

He also ended up on the president's infamous enemies list. In the 1980s and 1990s, he satirized and criticized Presidents Reagan, George H.W. Bush, and Clinton in addition to taking on the issues of the day: Gun control; abortion; the influence of fundamentalist Christian groups on public policy; and the Dot Com bubble. The tobacco industry was a favorite target of Herblock, who had smoked at one time. He gave it up and had criticized cigarette companies even before that.

Stating that he never got tired of his work, Herblock continued as the 21st century began by lampooning newly elected president George W. Bush. He died on October7, 2001, after a protracted bout of pneumonia six days short of what would have been his 92nd birthday. His final cartoon appeared in The Washington Post on August26.

Honors
Herblock won three Pulitzer Prizes for editorial cartooning (1942, 1954, 1979), shared a fourth Pulitzer Prize in 1973 for Public Service on Watergate, the Presidential Medal of Freedom (1994), the National Cartoonist Society Editorial Cartoon Award in 1957 and 1960, the Reuben Award in 1956, and the Gold Key Award (the National Cartoonists Society Hall of Fame) in 1979. In 1986 he received the Elijah Parish Lovejoy Award as well as an honorary Doctor of Laws degree from Colby College, and in 1999 an honorary Doctor of Arts degree from Harvard University.

In 1961, he was chosen as one of 50 outstanding Americans of meritorious performance in the fields of endeavor, to be honored as a Guest of Honor to the first annual Banquet of the Golden Plate in Monterey, California. Honor was awarded by vote of the National Panel of Distinguished Americans of the Academy of Achievement.

In 1966, he was selected to design the U.S. postage stamp commemorating the 175th anniversary of the Bill of Rights.

In 1987 he received the Four Freedoms Award for the Freedom of Speech.

In 2008 Herblock's work was the subject of exhibitions entitled Herblock's Presidents at the Smithsonian Institution's National Portrait Gallery (United States), and Herblock's History at the Library of Congress. In late 2009 and early 2010, the Library of Congress showcased a new exhibition called Herblock!. This exhibition included cartoons that represented Block's ability to wield his pen effectively and artfully. He used it to condemn corruption and expose injustice, inequality, and immorality. His topics included the Great Depression; the rise of fascism and World War II; communism and the Cold War; Senator Joseph McCarthy; race relations; Richard Nixon; the Reagan era; the 2000 election and more.

When Herb Block died in October 2001, he left $50 million with instructions to create a foundation to support charitable and educational programs that help promote and sustain the causes he championed during his 72 years of cartooning. The Herb Block Foundation awarded its first grants and the annual Herblock Prize in editorial cartooning in 2004. According to its website, the Herb Block Foundation "is committed to defending the basic freedoms guaranteed all Americans, combating all forms of discrimination and prejudice and improving the conditions of the poor and underprivileged through the creation or support of charitable and educational programs with the same goals. The Foundation is also committed to improving educational opportunities to deserving students through post-secondary education scholarships and to promoting editorial cartooning through continuing research."

On January 27, 2014, HBO premiered a documentary, Herblock: The Black & The White, which was executive produced by George Stevens Jr., produced and directed by his son, Michael Stevens, who also co-wrote with Sara Lukinson. The documentary interviews Jon Stewart, Lewis Black, Tom Brokaw, Bob Woodward, Carl Bernstein, Jules Feiffer, Ted Koppel and Ben Bradlee as witnesses to Block's life, work and indelible contribution to American satire.

Books of collected cartoons by Herbert Block

 * Block, Herbert. Herblock: The Life and Works of the Great Political Cartoonist ed. by Harry Katz (W. W. Norton, 2009), 304pp; prints more than two hundred fifty cartoons in the text; comes with a DVD containing more than 18,000 Herblock cartoons
 * Herblock's history: political cartoons from the crash to the millennium. Library of Congress, 2000.
 * Herblock: a cartoonist's life. Maxwell Macmillan International, 1993.
 * Herblock at large: "Let's go back a little ..." and other cartoons with commentary Pantheon Books, 1987.
 * Herblock through the looking glass Norton, 1984.
 * Herblock on all fronts: text and cartoons New American Library, 1980
 * Herblock special report Norton, 1974
 * Herblock's state of the Union. Simon & Schuster, (1972)
 * The Herblock gallery. Simon & Schuster, (1968)
 * Straight Herblock.  Simon & Schuster (1964)
 * Herblock's special for today. Simon & Schuster, (1958).
 * Herblock's here and now. Simon & Schuster, (1955).
 * The Herblock book (1952)
 * Herblock looks at Communism [1950?]

title: Herblock