British billionaire Richard Branson has unveiled plans to pilot a "flying" mini-submarine down to the furthest depths of the oceans, in his latest record-breaking adventure.

The single-seater Virgin Oceanic craft will try to reach the deepest points in each of the world's five oceans - in what would be the first such feat - starting with the deepest of them all, in the western Pacific later this year.

"With space long ago reached by man, and commercial spaceflight tantalisingly close, the last great challenge for humans is to reach and explore the depths of our planet's oceans," said Mr Branson.

"There are enormous amounts of the oceans that have not been explored. More men have been to the moon than have been down further than 20,000 feet," he said.

Mr Branson will share piloting duties with US sailor and explorer Chris Welsh, the chief pilot of the winged, snub-nosed submersible, in the five dives planned over the a two-year period.

If plans go well and pressure testing succeeds, the first will be taken by chief pilot Mr Welsh into the Mariana Trench in the Pacific, which goes down to 36,201 feet (11,033 metres), later in 2011.

Mr Branson, back-up pilot on the first trip, is then scheduled to pilot the red, white and blue submersible to the Atlantic's Puerto Rico trench, 28,232 feet (8,605m) deep, which has never been explored before.

The sub can "fly" with a "completely unique flying wing" for up to 10 kilometres at the bottom of the ocean, and operate autonomously for up to 24 hours, all the time recording high definition video of never-seen marine life.

Mr Welsh said the potential risks of the adventure are enormous: to withstand pressures of up to 1,000 times normal atmospheric pressure, the craft is made from 8,000 pounds of carbon fibre and titanium, with a quartz viewing dome.

"No leak is tolerable. A leak would cut through stainless steel or human flesh and mean certain death," he said. "The depth is beyond the capabilities of any other craft, so rescue is impossible.

"It's like being on the dark side of the moon," he added.

The project has a serious scientific aim, Mr Branson stressed, noting that partners include the renowned Scripps Institution of Oceanography which will be among its key science advisors.

"From a scientific point of view this is akin to discovering the Amazon for the first time. The amount that they're going to discover down there is incalculable.

"The scientists have just lined up to try to get involved with this project," he added.

- AFP