Lungs the Size of Lemons

From Alec Wilkinson's New Yorker piece on free diving, "a sport in which divers, on a single breath, descend hundreds of feet, into cold and darkness, and often pass out before they return":

What allows a person to hold his or her breath and dive to severe depths is an autonomic process called the mammalian diving reflex, which is activated when the nerves in the face come into contact with water, most effectively with cold water.... First, the heartbeat slows.... Under pressures of depth, blood withdraws from the arms and legs and concentrates in the chest. This is called the blood shift. Meanwhile, the lungs compress, halving themselves after ten metres, then reducing by degrees until, by a hundred metres, they are something like the size of a fist; free diving is the only sport in which the lungs shrink and the heart slows. The blood shift prevents the chest from collapsing....

"It takes a while to accept the physiological truths of a free dive, which are that your body knows how to conserve oxygen on its own," Campbell told me. "When you start, you don't necessarily believe that your body will take care of you at fifty and sixty metres. The reason you have to believe your lungs are the size of lemons is that, if you don't, the dive reflex won't kick in—it can be inhibited by stress.... In the early stages of learning to dive, it's very much 'What the hell am I doing?' You're getting to about six metres and racing back to the surface, because it feels so foreign. You're in a very irrational environment, being so far from the surface. It's human endurance, but you're doing it in a place where you shouldn't be able to prove human endurance."