Donald Walsh is stepping back in time at the National Museum of the United States Navy. DONALD WALSH: "Our job was to maintain and operate the bathyscaphe. The scientists at the Navy lab would decide what kind of research projects we have and what kind of equipment we put on it to make measurements and sampling under the sea." In 1960, Mr. Walsh was a young Navy lieutenant. He co-piloted a free-diving vehicle called the Trieste to the deepest part of the Pacific Ocean. DONALD WALSH: "What better demonstration of the safety of this platform than to go to the deepest place in the ocean and come back and perfectly intact and in working order." Mr. Walsh says the Trieste looks a little like a submarine. DONALD WALSH: "Basically, it is an underwater balloon. You've got two parts to it. You've got the balloon here -- which is this long, cylindrical object -- and that's filled with a lighter-than-water substance, which is aviation gasoline. Oil floats on water, so you get, you get buoyancy or lift. And then, beneath the balloon, like a balloon up in the air, you have a cabin for the fragile humans." There was just enough room for two people: Mr. Walsh and Swiss scientist Jacques Piccard. On January 23rd, 1960, the Trieste began its historic dive. Nine and a half kilometers down, the two men heard something unusual. DONALD WALSH: "It got our attention. But we, we didn't know at the time what it was. We just knew that we're still alive and everything was functioning well. All our instruments, indicators said that the dive was progressing just fine." The sound came from a crack forming in the window. Luckily, it did not leak and the Trieste arrived in one piece at the deepest point of the Mariana Trench. Until recently, no one had returned to that part of the ocean. JAMES CAMERON: "As soon as the sub is back on deck at the end of the dive ... " In March, filmmaker and explorer James Cameron reached the Mariana Trench alone, in a vessel he designed. JAMES CAMERON: "We know very little about the species that live down there. We know very little about the distribution of the biological communities. We don't know how these animals have adapted to living under this unbelievable pressure that exists down there." Mr. Cameron's dive owe much to the dive, a half-century before, by the crew of the Trieste. I'm Shirley Griffith.