"It is an extraordinary coincidence," writes English physiologist Frances Ashcroft, "that the highest peak on Earth is also about the highest point at which humans can survive unaided." A coincidence, to be sure, and, like many other milestones of the limits of human endurance, one known to us through the joint efforts of scientists, mountain climbers, explorers, and athletes. Ashcroft's book is a thoroughly engaging survey of those limits and their origins in the nature of things, of what happens to human beings in the most difficult environmental conditions. She writes, for instance, of why it is that astronauts have trouble standing after returning to Earth (because, in part, their leg muscles quickly atrophy outside of terrestrial gravity); of how the famed Japanese pearl divers condition themselves to attain such extraordinary underwater depths; of how and why the consumption of carbohydrates and caffeine can improve athletic performance; of why British children so easily suffer heat exhaustion on trips to such semitropical venues as, say, Disneyworld, whereas young Saudis can tolerate much higher temperatures (but would likely not thrive in an English winter).Backed by extensive field research--the author has climbed Mount Kilimanjaro, sweated it out in Japanese hot tubs, and run after her share of buses--as well as by a wealth of laboratory studies, Ashcroft's book is of great appeal to anyone who wishes to test the world's limits--or their own. --Gregory McNamee