Nathaniel Rich reports on the physical toll depth-diving has on humans:
Today it is an economic and even geopolitical necessity for oil companies, in order to maintain pipelines and offshore rigs, to send divers routinely to depths of a thousand feet, and keep them at that level of compression for as long as a month at a time. The divers who do this work are almost entirely male, and tend to be between the ages of twenty-five and forty. Were they any younger, they would not have enough experience or seniority to perform such demanding tasks. Any older, and their bodies could not be trusted to withstand the trauma.
A primer on the dangers involved:
[In 1962], it was widely believed that no human being could safely dive to depths beyond three hundred feet. That was because, beginning at a depth of one hundred feet, a diver breathing fresh air starts to lose his mind. This condition, nitrogen narcosis, is also known as the Martini Effect, because the diver feels as if he has drunk a martini on an empty stomach—the calculation is one martini for every additional fifty feet of depth.
But an even greater danger to the diver is the bends, a manifestation of decompression sickness that occurs when nitrogen gas saturates the blood and tissues. The problem is not in the descent, but the ascent. As the diver returns to the surface, the nitrogen bubbles increase in size, lodging in the joints, arteries, organs, and sometimes the brain or spine, where they can cause pain and potentially death. The deeper a diver descends, the more slowly he must ascend in order to avoid the bends.
If you’re looking for an excellent read on the subject, check out Robert Kurson’s Shadow Divers.