Cavitation is the process of formation of vapour bubbles of flowing fluid in a region where the pressure of the liquid falls below its vapour pressure and the sudden collapsing of these vapour bubbles in region of high pressure. At first small vapour filled bubbles are formed that gradually increase in size. As the pressure of the surrounding liquid increases, the cavity suddenly collapses-a centimeter sized cavity collapses in milliseconds. Cavities implode violently and create shock waves that dig pits in exposed metal surfaces. . At first, the physical characteristics of boiling and cavitation are almost identical. Both involve the formation of small vapour-filled spherical bubbles that gradually increase in size. However, the bubbles produced by the two processes end in very different manners. In boiling, bubbles are stable: the hot gas inside either escapes to the surface or releases its heat to the surrounding liquid. In the latter case, the bubble does not collapse, but instead fills with fluid as the gas inside condenses. When it acts upon propellers, cavitation not only causes damage but also decreases efficiency. The same decrease in water pressure that causes cavitation also reduces the force that the water can exert against the boat, causing the propeller blades to "race" and spin ineffectively. When a propeller induces significant cavitation, it is pushing against a combination of liquid water and water vapor. Since water vapor is much less dense than liquid water, the propeller can exert much less force against the water vapor bubbles. With the problems it causes, it is no wonder maritime engineers try to avoid cavitation.