A bathyscaphe (
The float is filled with gasoline because it is readily available, buoyant, and, for all practical purposes, incompressible. The incompressibility of the gasoline means the tanks can be very lightly constructed, since the pressure inside and outside the tanks equalises and they are not required to withstand any pressure differential at all. By contrast, the crew cabin must withstand a huge pressure differential and is massively built.
To descend, a bathyscaphe floods air tanks with sea water, but unlike a