watershedplus:

When a solid object falls in a pool, a sheet of liquid, the crown splash, is sent upward. Simultaneously, the object pulls a cavity of air down with it. As the water moves inward, this cavity is pinched. As the diameter of that pinched cavity shrinks, the velocity of the upward escaping air increases, resulting in the formation of an air jet moving faster than the speed of sound. Next time you throw a stone into a pond, enjoy the knowledge that you’ve broken the sound barrier.
From: D. van der Meer
Via f*#yeahfluiddynamics