The director of "Titanic," ''Avatar" and other films used a specially designed submarine to dive nearly seven miles. He spent time exploring and filming the Mariana Trench, about 200 miles southwest of the Pacific island of Guam, according to members of the National Geographic expedition.
Cameron returned to the surface of the Pacific Ocean on Monday morning local time, Sunday evening on the U.S. East Coast, according to Stephanie Montgomery of the National Geographic Society.
He spent a little more than three hours under water after reaching a depth of 35,756 feet before he began his return to the surface, according to information provided by the expedition team. He had planned to spend up to six hours on the sea floor.
Cameron's return aboard his 12-ton, lime-green sub called Deepsea Challenger was a "faster-than-expected 70-minute ascent," according to National Geographic.
The scale of the trench is hard to grasp - it's 120 times larger than the Grand Canyon and more than a mile deeper than Mount Everest is tall.
"It's really the first time that human eyes have had an opportunity to gaze upon what is a very alien landscape," said Terry Garcia, the National Geographic Society's executive VP for mission programs, via phone from Scotland.
Swiss engineer Jacques Piccard and Don Walsh, a U.S. Navy captain, are the only others to reach the spot. They spent about 20 minutes there during their 1960 dive but couldn't see much after their sub kicked up sand from the sea floor.
One of the risks of a dive so deep was extreme water pressure. At 6.8 miles below the surface, the pressure is the equivalent of three SUVs sitting on your toe.
Cameron told The Associated Press in an interview after a 5.1 mile-deep practice run near Papua New Guinea earlier this month that the pressure "is in the back of your mind." The submarine would implode in an instant if it leaked, he said.
But while he was a little apprehensive beforehand, he wasn't scared or nervous while underwater.
"When you are actually on the dive you have to trust the engineering was done right," he said.
The film director has been an oceanography enthusiast since childhood and has made 72 deep-sea submersible dives. Thirty-three of those dives have been to the wreckage of the Titanic, the subject of his 1997 hit film, which is being released in a 3-D version next month.