"I"ve been a hardcore mountain climber for 20-25 years, and when I"d pretty much done many of the things I wanted to do there, I was looking for a different challenge and I viewed it as a nice symmetrical thing to do, let"s go to the deep oceans," he tells BBC Future from his home in Texas. "And it turned out that no-one had been to the bottom of all five of the world oceans. They"d never even been to the bottom of four of them."