Located in the western pacific, the Mariana Trench is the deepest part of the ocean, plunging down 11km.

Down there it's pitch black, icy cold and the pressure is immense.

Now explorers with funding from the private sector are planning to return to the bottom of the Trench, for the first time for over 50 years.

Rebecca Morelle meets Jim Gardner, who works for the US Center for Coastal and Ocean Mapping, and has just completed the most detailed survey ever of the Mariana Trench, using sonar.

Alan Jamieson, an ecologist at Oceanlab at the University of Aberdeen in Scotland, uses remote controlled submersibles to study the animals and plants that live at extreme pressure in the deepest parts of the oceans.

He tells Rebecca why he believes it is preferable to deploy robots rather than humans to do this research.

Legendary marine biologist and underwater explorer, Sylvia Earle, argues that it is essential for us to visit the depths of the ocean and see the extraordinary environment with our own eyes.

As the former science chief for the National Oceanic and Atmospheric Administration, NOAA - the ocean's equivalent of NASA - Sylvia Earle says that the seas have always been the poor relation to space.

Rebecca finds out from Bill Raggio of precision glass company Rayotek in San Diego, how to build a glass sphere for Triton submarines which will stop the three-man crew from being crushed by the pressure a the bottom of the Mariana Trench.

And Sandra Brook from the Marine Conservation Biology Society talks about how research scientists may work with the commercial teams, like Triton, in the future as resources dry up for purely research submersibles.