His history-making solo venture to Challenger Deep, part of the Mariana Trench in the western Pacific Ocean, left him feeling "complete isolation from all of humanity," he said.
"I felt like I literally in the space of one day have gone to another planet and come back."
At more than 10,900 meters (about 35,800 feet), the Mariana Trench is deeper than Mount Everest is tall. It has had only two previous human visitors:U.S. NavyLt. Don Walsh and the late Swiss explorer Jacques Piccard, who descended to that spot in 1960.
Cameron is the first to make the trek alone. And he did so hours before he was due in London for the premiere of the 3-D version of his 1997 blockbuster movie, "Titanic."
The man who was also behind "Avatar" went down in a high-tech vessel, the Deepsea Challenger, which he and a group of scientists and engineers constructed in Australia over the past eight years.
The bottom was a "very soft, almost gelatinous, flat plane, almost featureless" and went as far as he could see, Cameron said, describing the "vast frontier down there."
"It's a completely alien world," he said.
But he did not see the kind of large life forms that some might dream up, he said.
"We'd all love to think there are giant squid and sea monsters down there. We can't rule it out, but my bet is there aren't. What you're going to find is these very, very interesting animals, the likes of which we've never seen before, that have adapted to this extreme environment."
In a video feed afterward from a large ship, Cameron told reporters, "I had this idea that life would adapt, life would be able to adapt to the deepest place. But I don't think we're seeing that. At a microbial level, yes, life is adapting."
He said he did not see fish at such depth. "The only free swimmers I saw were these little ... shrimplike animals," scavengers that devour potential food that falls, such as dead fish or whales.
It's a cold, dark, "completely black world that's devoid of sunlight" and warmth, he said.
Animals that far down are usually white with no pigment, he said. Some have eyes to see bioluminescence, while others have no eyes.
The full trip took about seven hours, he said.
While the journey went well, one technical glitch caused a problem, he said. The hydraulics prevented him from using a manipulator arm to collect samples to bring back to the surface. Cameron said he's confident it will be fixed for the next trip.
This trip is only the beginning, he said.
Horrific tsunamis originate in the depths of the ocean, he said, suggesting that a better understanding could ultimately help societies understand and prepare for such disasters.