The director of "Titanic" and "Avatar" traveled to the south Pacific's Mariana Trench, nearly seven miles underwater, in a specially-designed submarine.
The trench is the Earth's deepest point. The expedition was part of a National Geographic Society project. Cameron reached a depth of 35,756 feet and stayed on the bottom for about three hours before he began his return to the surface.
"It was bleak. It was like the moon," said Cameron. "I'm thinking, 'Man this is a long way down.'"
Cameron captured still photographs and video, but there was no immediate word on when the images will be released. The Geographic said the expedition is being chronicled for a 3-D feature film for theatrical release and subsequent TV broadcast.
Cameron is only the third man to reach the bottom and the first to do it solo.
The Associated Press contributed to this report.