Krack had uncovered a technique for triggering the mammalian diving reflex, a reflex that optimizes respiration and, like dolphins, whales, and some birds, allows us to operate underwater for extended periods of time. Here’s how it works: When the nerves of the human face come in contact with water, our heartbeat begins to slow (10 to 30 percent in amateurs; up to 50 percent in professionals). A slower heart rate requires less oxygen, leaving more left over for other organs. Next, as pressure from depth increases, blood leaves our extremities—first fingers and toes, next hands and feet,
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