Designing a submarine that 'flies' to the Mariana deep

Four teams are diving to the deepest part of the ocean, the Mariana Trench, which lies 11km (7 miles) down in the western Pacific.

One of them is a team backed by Virgin Oceanic, the latest addition to Richard Branson's empire.

The vehicle is called the Deep Flight Challenger and will "fly" through the water like a plane flies through the air.

Here, engineer Graham Hawkes describes why he has designed the vessel in this way.