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.