A handful of sand dunes around the world make eerie low-pitched noises when sand slides down their faces. The “singing” can be a deep hum, a long bass note, or even a roar - and it can carry for several kilometres across the desert.
The sound comes from millions of grains rubbing against each other in just the right way. The grains have to be a very specific size and shape and the layer underneath has to be firm enough to vibrate. When those conditions line up, the whole avalanche acts like a loudspeaker.
Marco Polo heard the booming dunes of the Gobi over 700 years ago and thought they were demons trying to lead travellers astray. Today scientists know it’s just physics, but it still sounds genuinely spooky if you’ve never heard a dune before.
