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 and hypoxic breathing gas is required below 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 . However, the Professional Association of Diving Instructors (PADI) defines anything from 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 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 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 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 . 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 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 an simulated depth of .
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) carries a risk of high-pressure nervous syndrome. Coping with the physical and physiological stresses of deep diving requires good physical conditioning.