How can a sub dive all the way down?
Hollywood director James Cameron may be close to making a dive to the deepest place on Earth.
In a one-man submarine, Deepsea Challenger, he plans to journey 11km (seven miles) down beneath the waves to the bottom of the Mariana Trench, in the western Pacific.
Here, the team's project manager David Wotherspoon reveals how the sub has been put together.
Click to see more of the BBC's special report on the race to the bottom of the ocean.
Produced by the BBC's Rebecca Morelle and Simon Hancock.
- From the section Science & Environment