A bathyscaphe (icon or ˈ) is a free-diving self-propelled deep-sea submersible

Submersible

A submersible is a small vehicle designed to operate underwater. The term submersible is often used to differentiate from other underwater vehicles known as submarines, in that a submarine is a fully autonomous craft, capable of renewing its own power and breathing air, whereas a submersible is...

, consisting of a crew cabin similar to a bathysphere, but suspended below a float rather than from a surface cable, as in the classic bathysphere design.

The float is filled with gasoline because this is readily available, buoyant, and for all practical purposes, incompressible. The incompressibility of the gasoline means the tanks can be very lightly constructed as the pressure inside and outside of the tanks equalises and they are not required to withstand any pressure differential at all.