Why do certain sand dunes "sing" or hum? When Marco Polo heard it in China, he suspected evil spirits. Charles Darwin heard it too, but couldn't explain the origin of the strange sound.
Scientists today call it 'singing sand' — the noise that sand grains make as they shuffle down the slopes of some sand dunes — a deep, groaning hum at the bottom half of a cello's range that can be heard for miles. The music can be heard as a sand avalanche is triggered by the wind or even by people shearing sand away with their hands and feet.
In 2009, University of France researchers found that it's not the motion of the sand dune, but rather the size of the sand grain, that creates the sound. The notes depend on the size of the grains and the speed at which they whistle through the air. As the grains move together down a dune, a constant stream of collisions is created. Each bump on its own would be inaudible — but add them together and you get the sound of millions of little collisions in unison.
Singing sand dunes are rare — it's estimated that there are approximately 30 dunes around the world that have the right grain size and dry conditions — each with its own distinctive pitch.