"On any one of these trips into the twilight, the rate of new-species discovery can top 10 per hour. You can imagine why a diver might be tempted to stay as long as possible, to linger and simply take in this cold, bizarre, light-starved world. But there are other forces at work, namely the desperate need to manage risk in this inherently risky endeavor and to keep decompression time down to a reasonable number of hours.

There’s a lot more to the risks of deep diving than the obvious distance it puts between the diver and that unlimited supply of air at the surface. The pressure that goes hand in hand with depth does bizarre things to one’s physiology, particularly in relation to the air we breathe. Every 33 feet of depth stacks one whole atmosphere’s worth of pressure on top of a diver. That pressure compresses everything, gases in particular—so much so that a lungful of air at 500 feet contains 12 times as many molecules as the same breath at the surface. That’s 12 times the number of molecules pushing into a diver’s tissues and being absorbed by the blood, which is almost exactly 12 times too many.

At depth, the 21-percent concentration of life-giving oxygen we breathe on land quickly becomes toxic and seizure-inducing. The other chief component of air, nitrogen, starts to have a narcotic effect somewhere beyond about 90 feet. Seizures and narcosis are two conditions you most definitely do not want hundreds of feet underwater. To prevent both, the divers use an advanced breathing system that dilutes the standard percentages of oxygen and nitrogen with helium, an inert gas that has none of the toxicity and narcotic effects of the other two gases. This closed-circuit system, known as a “rebreather,” also scrubs carbon dioxide from air the diver exhales and recycles oxygen, which allows for longer dive times. A sophisticated onboard computer monitors the mix of gases in real-time and meters out just enough oxygen to keep a diver conscious, clear-headed, and seizure-free."

Steven Bedard is Senior Science Editor at the Califiornia Academy of Sciences.