Branson congratulated Cameron on making the solo submarine dive into the Mariana Trench, the deepest spot in the world, earlier Monday.

"It's incredible what he achieved today," Branson said in a telephone interview from his private Caribbean retreat, Necker Island.

The director of "Avatar" and "Titanic" is now the leading man in a rarefied race among entrepreneurs and explorers to get to the world's almost entirely unexplored ocean beds. The Mariana had only been reached once before in a brief visit by a two-man crew in 1960.

Branson's team has also been planning to visit the Mariana, as part of a mission to reach the deepest spots in each of the Earth's five oceans.

But Branson is not far behind Cameron in the race to the bottom.

His Virgin Oceanic submarine, an airplane-like vehicle that would not look out of place in a James Bond film, will undergo final pressure testing before taking Branson later this year to the bottom of the Atlantic, in the Puerto Rico Trench.

That won't be as deep as the approximately 36,000 feet, or 11,000 meters, that Cameron's Deepsea Challenger reached in the Pacific. But it's never been explored by a manned submersible and is a landmark in the world's most mysterious terrain.

"Nobody in the Atlantic has been further down," Branson said, "and the Puerto Rico Trench is nearly 30,000 feet (8,600 meters), deeper than Everest is high."

The founder of the Virgin business empire said he's also looking forward to pairing his craft with Cameron's more torpedo-like submarine.

"It's quite possible we might put the two submarines together and explore different parts of the oceans," Branson said. "They're the only two submarines in the world capable of going below 18,000 feet."

The difference, he said, is that Cameron's performs better on the dive and carries more film and other equipment, whereas his comparatively lightly built Virgin Oceanic is best suited to exploring once submerged.

"Ours can go quite big distances at the bottom of the ocean," he said, looking forward to uncovering Spanish galleons or "a species that hasn't been discovered."

The subterranean landscape he described was not unlike the surface of the moon.

It was a "very lunar, very desolate place. Very isolated," Cameron said.

"I felt like I in the space of one day had gone to another planet and come back," he said, describing the ocean floor as a "completely featureless, alien world."

The acclaimed film director described the experience of hurtling down the "yawning chasm" of the ocean: "Falling through darkness -- that's something that a robot can't describe."

Cameron descended about seven miles (11 kilometers) to the bottom of the Mariana Trench in the western Pacific, where temperatures are barely above freezing and the pressure is a crushing thousand times that at sea level.

He told reporters in a phone press conference that he was at the bottom of the ocean for about a little more than two and a half hours, and had to cut short the planned stay of six hours because of problems with the ocean craft's hydraulics system.

The voyage was the first manned expedition to the trench in more than half a century and Cameron said it was the culmination of more than seven years of planning.

The journey down to the Challenger Deep valley of the Mariana Trench, which lies southwest of Guam, took two hours and 36 minutes, organizers said.

The filmmaker collected samples for research in marine biology, microbiology, astrobiology, marine geology and geophysics, and captured photographs and 3D moving images.

Cameron is the first person to make a solo dive to the Pacific Ocean trench. The last dive of any kind there was made by a two-man team in a relatively brief expedition back in 1960.

Mission partner the National Geographic said Cameron had reached a depth of 35,756 feet (10,898 meters) at 7:52 am Monday (2152 GMT Sunday) in his specially designed submersible the Deepsea Challenger.

Cameron said in the telephone news conference that being able to make the journey was "the culmination from my perspective of a lifelong dream," and that he hopes to be able to continue to marry his love of exploring the depths of the sea with his work as as director.

"Hopefully I don't have to choose between them. Hopefully I can do the two in parallel," he said. "I don't want to have to decide."