A solid object impacting on liquid creates a liquid jet due to the collapse of the impact cavity. Using visualization experiments with smoke particles and multiscale simulations we show that in addition a high-speed air-jet is pushed out of the cavity. Despite an impact velocity of only 1 m/s, this air-jet attains supersonic speeds.
The cavity pressure nonetheless is merely 1.02 atmospheres and thus much lower than the pressures required for supersonic flow through a rigid nozzle. The high air speeds are shown to result from the "nozzle" being a liquid cavity shrinking rapidly in time.
Stephan Gekle, Ivo Peters, Jose Manuel Gordillo, Devaraj van der Meer and Detlef Lohse in their paper: Supersonic Air Flow due to Solid-Liquid Impact via Slashdot.
Does that mean the deeply satisfying "bloop" that a stone makes as it hits the water is actually a sonic boom?