Scuba diving
BCD (buoyancy control device) to control whether the diver floats or sinks.  The diver can add air from an inflator valve or remove air by opening a dump valve. Divers may also wear weights to stop themselves from floating upwards when their diving suit is too buoyant. The BCD lets the diver adjust their buoyancy so they can be neutral (float in place) at a depth they want.  If the diver swims upward the air in the BCD will expand because the water pressure is less, and the diver must let some air out or float to the surface. If the diver swims down the air will compress and the diver must put in more air or sink to the bottom, so the diver must carefully control how much air is in the BCD whenever their depth changes.