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Shaw did not resurface after an operation to recover the remains of Deon Dreyer who drowned in the world's third deepest freshwater cave in 1994.
The bodies of both divers were recovered by police divers two days later. Shaw had expressed the wish that his ashes be strewn at the Boesmangat site.
But how did Shaw die? According to Cruise, the possibilities are endless.
A combo of dangerous gases
Technical divers make use of a mix of oxygen, helium, and sometimes a bit of nitrogen. Both the depth of the proposed dive and the specific circumstances determines this mixture.
These gases, all of which can be lethal in the wrong amounts, are placed in a breathing apparatus that the diver carries with him as he descends. To avoid nitrogen-induced decompression sickness, the divers don't dive on normal air, which has a nitrogen level of about 75%.
A diver like Shaw would have spent months before the Boesmangat expedition to figure out which combination of gases would be ideal.
"Either he got his calculations wrong or his rebreather (a device that recycles rather than vents exhaled breath) fed him the wrong stuff," Cruise says. For this, human error could have been to blame.
"When you're diving at great depths, you don't necessarily have all your wits about you. The gases could affect you in various ways and you can't think clearly," Cruise says. It is therefore quite possible that Shaw adjusted his gas feed to a fatal level while he was exploring the depths of the cave.
Too much oxygen would have knocked him out instantly, nitrogen could have led to narcosis (a state of diminished consciousness) and a helium overdose could have induced so-called high-pressure nervous syndrome. This is characterised by tremors, nausea, dizziness, as well as decreased motor and mental performance.
Boesmangat particularly treacherous
What makes diving in Boesmangat particularly dangerous is the fact that the freshwater cave doesn't go straight down. This makes it virtually impossible to pull a diver up, if he should experience problems, Cruise says.
Pulling Shaw up would in itself have been a risky procedure. On return to normal atmospheric pressure, the nitrogen dissolved in his bloodstream could have expanded to form bubbles. This causes pain and blocks the circulation in the small blood vessels in the brain and elsewhere – a condition known as decompression sickness.
Poor visibility in the cave could have been another problem for Shaw, Cruise reckons. While the water in freshwater caves is usually quite clear, a slight movement is all that is needed to disturb the sediment below. A diver could easily get lost in the murky water. Shaw disappeared at a depth of 270 m.
The fact is, "diving beyond 30 m is extremely dangerous," according to Cruise. Those few divers that actually succeed in surviving deep diving expeditions often show signs of brain damage afterwards. Their life span is generally also much shorter.
Watch: David Shaw's last dive
