Supercavitation is the phenomenon that occurs when a body moving through a fluid (usually water) reaches a speed high enough to form a low-pressure, fluid-free1 pocket about itself due to the bowshock forcing water to the sides hard enough to leave a near-vacuum immediately surrounding the hull. This is useful for making small (or perhaps large) vehicles that can travel at high speed underwater. See cavitation torpedo for some of the advantages and disadvantages of this approach.

1. nadir_acme points out, quite correctly, that air is a fluid. I should emphasize that these pockets or voids can in many cases be near-vacuum, not in fact gas-filled, so my statement really should be 'free of everything.'

Super Cavitation is the phenomenon, whereby an object travelling underwater, moves at such a speed that the low pressure area immediately behind the nose of the object cavitates to produce a pocket of low pressure water vapour which completely envelops the object.

This frees the object of the drag associated with travel through water (approximately 1000 times greater than air), theoretically enabling the object to achieve supersonic speeds.

It has been proposed that supersonic submarines could be produced for rapid transatlantic travel.

While this technology has not yet been applied to large vehicles, both Russia and the USA have produced super cavitation torpedoes. These are powered by rocket engines (the cavitation bubble must completely envelop the vehicle, thus rendering propellers unusable) and acheive initial acceleration by venting rocket exhaust gases through the nose cone.

Unfortunately, a side-effect of the super cavitation bubble's need to completely surround the object, is that steering devices can not be used. Thus the torpedo behaves more like a bullet, than a steerable torpedo.
However the speed of such devices renders it impossible to take evasive action or use counter-measures.
It is widely rumoured that the Kursk tragedy was initiated by the failure of a super-cavitation torpedo rocket engine whilst still inside the submarine.

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