Pressure changes cause a jet of air to be expelled at supersonic speeds (Image: Stephan Gekle/PRL)

IS IT a bird, is it a plane? No, it’s a rock falling into a pool of water, but the jet of air it produces flies faster than a speeding bullet.

When an object such as a pebble drops into water, an air-filled cavity is created which ejects air at supersonic speeds, discovered Stephan Gekle at the University of Twente in the Netherlands, and colleagues.

Using high-speed photography, the team spotted a cavity of air forming in an hourglass shape – with the top …