Chris Welsh in Deep Flight Challenger
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Meet the man diving alone to the Mariana Trench

22 February 2012 Last updated at 14:43 GMT

Four teams are diving to the deepest part of the ocean, the Mariana Trench, which lies 11km down in the west Pacific.

One effort is backed by Virgin Oceanic, the latest addition to Richard Branson's empire.

Their vehicle is called the Deep Flight Challenger - it is a submersible that fits just one person.

Chris Welsh explains why he wants to take up this lonely challenge.