Bryson opens by giving the statistic that “no less than 99.5 percent of the world’s habitable space by volume, according to one estimate, is fundamentally—in practical terms—off-limits to us” (239). He is talking about how inhospitable most of Earth is for humans considering that most of the planet is covered by oceans. Because water is 1,300 times heavier than air, humans can’t withstand the water pressure of great depths. In fact, the record for deepest free diving is 236 feet, which is quite a feat for a human but barely scratches the surface of the oceanic abyss.

Bryson states that since the air we breathe is 80 percent nitrogen, when the human body goes under pressure, the “nitrogen is transformed into tiny bubbles that migrate into the blood and tissues” (241). If a diver ascends too quickly, these trapped bubbles begin to fizz, clogging the blood vessels and causing immense pain. This is called “the bends.” The only way to avoid the bends is to stay for only the briefest time at the depths, which prevents the nitrogen in the body from dissolving, or, alternately, to ascend in careful stages.

Bryson states that a lot of what we know about surviving in extreme states comes from father-and-son team John Scott and J.