Deep diving

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.

For instance early experiments carried out by COMEX using heliox and trimix attained far greater depths than any recreational technical diving.

The latter avoids the high-pressure nervous syndrome (HPNS) caused by helium and eases breathing due to its lower density.

[26] 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][page needed] 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.

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.

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.

[citation needed] Due to the short bottom times and long decompression, scuba dives to these depths are generally only done for deep cave exploration or as record attempts.

[citation needed] 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.

Serious issues that confront divers engaging in ultra-deep diving on self-contained breathing apparatus include: In addition, "ordinary" risks like size of gas reserves, hypothermia, dehydration and oxygen toxicity are compounded by extreme depth and exposure and long in-water decompression times.

[citation needed] 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.

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.