A cavity (white) forms behind an object (black) passed by a rapidly flowing liquid (blue)
Supercavitation is the use of cavitation effects to create a large bubble of gas inside a liquid, allowing an object to travel at great speed through the liquid by being wholly enveloped by the bubble. The cavity (the bubble) reduces the drag on the object, since drag is normally about 1,000 times greater in liquid water than in a gas.
Current applications are mainly limited to very fast torpedos. Some experiments with small submarines also exist.