Submarine gives taste of Mariana Trench dive
22 February 2012 Last updated at 19:51 GMT
Four teams are diving to the deepest part of the ocean, the Mariana Trench, which lies 11km (seven miles) down in the western Pacific.
One of them is Triton submarines, a Florida-based company.
Their team has been in the Bahamas to test out a prototype submersible, which their full-ocean-depth model will be based on.
Science reporter Rebecca Morelle joined them for the sub's first night dive, which was piloted by Patrick Lahey.