Writer James Nestor explores the science of the “mammalian dive reflex,” the phenomenon by which water triggers an immediate decrease in heart rate.

In 1949, a stocky Italian air force lieutenant named Raimondo Bucher decided to try a potentially deadly stunt off the coast of Capri, Italy. Bucher would sail out to the center of the lake, take a breath and hold it, and free-dive down one hundred feet to the bottom. Waiting there would be a man in a diving suit. Bucher would hand the diver a package, then kick back up to the surface. If he completed the dive, he’d win a fifty-thousand-lira bet; if he didn’t, he would drown.

Scientists warned Bucher that, according to Boyle’s law, the dive would kill him. Formulated in the 1660s by the Anglo-Irish physicist Robert Boyle, this equation predicted the behavior of gases at various pressures, and it indicated that the pressure at a hundred…