Cavitation, seen here created by a propeller, is when fluid under force develops an area of low pressure lower than the liquid’s vapor pressure. This causes the liquid to convert to gaseous form in these areas and create a cavity that rapidly collapses. This collapse generates a small shock wave that causes significant damage to metal via metal fatigue and as such cavitation is usually undesirable. However cavitation isn’t always bad and has lead to a more efficient method of marine transport using super-cavitation. In super-cavitation the object moving through the water develops such a large low pressure area that it becomes enveloped in the vapor phase, significantly lowering skin drag.