7 hours ago, Bill W said:

To descend, a bathyscaphe floods air tanks with sea water.  The tanks of water on a submarine cannot be displaced with compressed air to make the submarine rise, though.  That's because water pressure at the depths at which the submarine operates are too great.   

The design is ingenious. The largest part, the float on top, is filled with buoyant gasoline. Hoppers, open at the bottom, are filled with iron shot held by electromagnets. When released, the weight reduction allows the craft to surface and it's a failsafed system.

Auguste Piccard, its inventor, coined the word by combining bathy with a form of the Greek word for ships hull.