As a shape passes through water at an increasing speed, the pressure that holds the water to the sides and back of the shape is lowered. Depending upon the water temperature, when the pressure reaches a sufficiently low level boiling (i.e., the formation of water vapor) will begin. The collapsing action, or implosion, of the bubbles releases energy that chips away at the blades causing a "cavitation burn," or erosion to the metal. The initial cause of the low pressure may be nicks in the leading edge, too much cup, sharp leading edge corners, improper polishing, or, sometimes poor blade design. Massive cavitation by itself is rare and is usually caused by a propeller that is severely bent or has had its blade tips broken off. This results in a propeller that is far too small in diameter for the engine.