A signal, poetic trait of leatherbacks is that "they can't comprehend barriers," Jones says. Their universe is uncontained. Put them in a tank, and they will ram the wall repeatedly until they die. So Jones invented a Jolly Jumper–like harness made of rubber tubing that suspended the leatherbacks in the middle of the tank, free to swim and dive but touching nothing. The turtles accepted the illusion that they were in the open sea. "People ask me, 'When did they get used to the tank and stop swimming?' " Jones says, "and the answer is: never." For three years they swam on, those big flippers churning slow figure eights. Jones and his research partners were the first to uncover how much food leatherbacks must consume, how quickly they grow, and when they reach sexual maturity. Because until we understand how the leatherback lives, we won't be able to stop what's killing it.