by ButterflyEffect

The primary task of a submersible is to not implode. The second is to reach the surface, even if the pilot is unconscious, with oxygen to spare. The third is for the occupants to be able to open the hatch once they surface. The fourth is for the submersible to be easy to find, through redundant tracking and communications systems, in case rescue is required. Only the fifth task is what is ordinarily thought of as the primary one: to transport people into the dark, hostile deep.