Drop a rock on a pond and watch the splash – but look carefully, because some of the air in the splash comes out faster than sound. Stephan Gekle of the University of Twente and colleagues carefully studied what happens when a 2 cm disc is pulled through water at just 1 m/s. The cavity produced in the water rapidly forms into an hourglass shape and air forced through the narrow “waist" can exceed the speed of sound even though it is driven by a pressure difference of just 2% of an atmosphere. At times, the dynamically evolving cavity resembles nozzles used for supersonic jet engines, but the way that it changes in time makes it a completely unique physical system.