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The Deep Flight Challenger submersible has been backed by Richard Branson's Virgin empire, and is currently undergoing its first water trials.

It was originally designed and built for billionaire adventurer Steve Fossett, who wanted to make the world's first solo dive to the bottom of the Mariana Trench.

But after he died in a plane crash, former property investor Chris Welsh, who is based in California, bought the submarine and is now taking on the lonely challenge himself.

Chris said: "I've been working day and night for two years pursuing this. I'm really excited to get out there and do it - I can't wait.

"The rewards of this is breaking ground in so many ways - in raw human exploration, on bringing back science, and in getting a better understanding of our world."

The experimental sub was created and built by British engineer Graham Hawkes.

Fitted with wings and a tail fin, its design is based on that of a plane.

Chris Welsh hopes to "fly" down to the bottom of the trench, and break the record for the first lone dive there, later this year.

After this, he and the Virgin team also plan to plunge down four other trenches around the world, with the hope of reaching the deepest point in five oceans.