How to build a deep-diving submarine: The glass
Four teams are diving to the deepest part of the ocean, the Mariana Trench, which lies 11km (7 miles) down in the west Pacific.
One of them is DOER Marine, a marine technology company based in San Francisco.
They are currently putting the finishing touches to the design of their Deep Search submersible.
Here, Liz Taylor, who runs the company, explains why glass is a crucial material for their sub.