Private firms are already competing to see who will dominate the burgeoning private space flight industry, and now the very wealthy have begun investing in private vessels to explore the other great frontier: the ocean's depths.

People with an awe for the unknown and a substantial amount of money to spend, including the director James Cameron and the airline titan Richard Branson, have been financing mini-submarines to ferry them to some of the most remote places on the planet, seafloors as much as seven miles below the surface.

"When I was a kid, I loved not only amazing ocean exploration but space, too," Cameron told the New York Times. "I can think of no greater fantasy than to be an explorer and see what no human eye has seen before."

Cameron has an established fascination with the submerged world, commissioning a submarine to help to film his Avatar 2, offering to dispatch some of his fleet of private submarines to help clean up the Gulf of Mexico oil spill and voyaing to the ruins of the Titanic (the subject of another one of his films), two miles deep.

But the development of new tecnhologies that are lowering the cost of building submarines is leading Cameron and others to refocus their efforts on a new underwater prize: the Challenger Deep, the darkness-shrouded floor of a canyon deep below the surface of the Pacific where bizarre creatures thrive far from the light of the sun. The Challenger Deep is part of a labyrinthe of deeply submerged chasms fracturing through the Pacific seafloor and it has only been seen once, by a U.S. Naval ship.

Branson, who has made forays into space with an offshoot of his airline empire called Virgin Galactic, announced his development of an 18-foot long solo deep sea submarine by calling deep sea exploration the "last great challenge for humans." But it is not just wealthy individuals who are getting into the deep-sea race; the firm Triton Submarines has begun constructing submarines with the aim of charging $250,000 per person for tours of the Challenger Deep.

"It's more an iconic experience than 'Gee, everything was so beautiful,' " said Mike McDowell, an organizer of adventure tools who has spoken with Triton. "And you eliminate a lot of people on the fear factor."