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 metres (98 ft) and hypoxic breathing gas is required below 60 metres (200 ft) 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 metres (98 ft). However, the Professional Association of Diving Instructors (PADI) defines anything from 18 to 30 metres (59 to 98 ft) as a "deep dive" in the context of recreational diving (other diving organisations vary), and considers deep diving a form of technical diving.^[1] In technical diving, a depth below about 60 metres (200 ft) 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 metres (1,640 ft) in 1977.^[2][3]

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 metres (1,750 ft) 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.^[2][4][5] 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 700 metres (2,300 ft).^[6] 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 610 metres (2,000 ft) in an ADS.^[7][8]

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 metres (2,300 ft).^{[2][9][10][11][12]}

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.^[25] 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.^[1] 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.^[1] 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) carries a risk of high-pressure nervous syndrome. Coping with the physical and physiological stresses of deep diving requires good physical conditioning.^[26]

Using open-circuit scuba equipment, consumption of breathing gas is proportional to ambient pressure – so at 50 metres (164 ft), where the pressure is 6 bars (87 psi), a diver breathes six times as much as on the surface (1 bar, 14.5 psi). 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 metres (20 ft) may be able to dive for many hours without needing to do decompression stops. At depths greater than 40 metres (131 ft), 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.^{[citation needed]}

• 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 lockout 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 reclaim systems 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.

Ultra-deep diving

Mixed gas

Amongst technical divers, there are divers who participate in ultra-deep diving on scuba below 200 metres (656 ft). 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 metres (790 ft) on self-contained breathing apparatus recreationally.^{[20][27][nb 4][nb 5]} The "Holy Grail" of deep scuba diving was the 300 metres (980 ft) 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". Unsurprisingly, 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^[24] (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^[28]) and Nuno Gomes reported short to medium term hearing loss.^{[29][unreliable source?]}

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,^[30] 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.^{[citation needed]}

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.^{[citation needed]}

Air

A severe risk in ultra-deep air diving is deep water blackout, or depth blackout, a loss of consciousness at depths below 50 metres (160 ft) 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.^[70] 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.^[71] 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.^[72] 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.^[27]

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 (394 ft) before failing to return line signals. He became the first diver to perish using an Aqua-Lung.^[78][79][80]
• Hope Root died on 3 December 1953 off the coast of Miami Beach trying set a deep diving record of 125 m (410 ft) with an Aqua-Lung; he passed 152 m (500 ft) and was not seen again.^[81]
• Archie Forfar and Ann Gunderson died on 11 December 1971 off the coast of Andros Island, while attempting to dive to 146 m (479 ft), 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 (400 ft), causing him to start an uncontrolled ascent before being intercepted by a safety diver at a depth of around 91 m (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 142 m (465 ft) when the narcosis very nearly overcame him as well. The bodies of Forfar and Gunderson were never recovered.^[27]
• Sheck Exley died in 1994 at 268 m (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.^[44]
• Dave Shaw died in 2005 in an attempt at the deepest ever body recovery and deepest ever dive on a rebreather at 270 m (886 ft).^[82][83]
• Brigitte Lenoir, planning to attempt the deepest dive ever made by a woman with a rebreather to 230 m (750 ft), died on 14 May 2010 in Dahab while ascending from a training dive at 147 m (482 ft).^[84]
• Guy Garman died on 15 August 2015 in an unsuccessful attempt to dive to 370 m (1,200 ft).^[85][86] The Virgin Island Police Department confirmed that Guy Garman's body was recovered on 18 August 2015.^[87]
• Theodora Balabanova died at Toroneos Bay, Greece, in September 2017 attempting to break the women's deep dive record with 231 m (758 ft). She did not complete the decompression stops and surfaced too early.^[88]
• Wacław Lejko attempting 275 m (902 ft) in Lake Garda, died in September 2017. His body was recovered with a ROV at 230 m (750 ft).^[88]
• Adam Krzysztof Pawlik, attempting a 316 m (1,037 ft) dive in Lake Garda, died on 18 October 2018. His body was located at 284 m (932 ft).^[89]
• Sebastian Marczewski reached the target depth of 275 m (902 ft) in Lake Garda but his tanks became entangled in his ascent line at 150 m (490 ft).^{[citation needed]} He died on 6 July 2019.^[90]