Thirty years ago, Bob Ballard discovered the wreck of the Titanic. He could have stopped there. Yet today, at seventy-three, he remains the world's most vigorous ocean explorer. ... Money is a frequent topic of conversation for Ballard, because it takes $10 million annually to keep the Nautilus in the water for four to six months every year. (Similar vessels, he says, cost six times that.) Ballard's unique position as an oceanographer/owner makes him both liberated and beholden. For that money that frees him to explore, Ballard must regularly court private donors, corporate sponsors, and politicians who believe in his mission enough to fight for public dollars. The donors love to hear about the Titanic. The politicians are guided by an interest in everything from Ballard's commitment to education to his exploration of the earth's crust. ... Ballard seems prouder of his discoveries of the prizes for which he was not looking—the prizes you win, he says, by spending "time on the bottom." In 1977 he discovered the very existence of hydrothermal vents, hot springs in the ocean floor near where tectonic plates move apart from one another, releasing a steady flow of superheated water from deep in the earth's crust. The water is a chaotic mess of mineral-rich fluids including sulfide that, when discharged into the frigid, pressurized water on the ocean floor, can create new ecosystems hospitable to a wild mix of creatures. In the worlds of marine biology and geology, it was a monumental discovery. In the actual world, nobody much noticed. ... "I got addicted to it. In my fifty-five years of exploring, how much of the ocean floor have I seen?" He holds his thumb and forefinger close together. "In all my discoveries, just that much." His eyes grow wide. "So how much have I not seen?"