Singing Sand Dunes Explained

When Marco Polo heard it in China, he suspected evil spirits. When residents of Copiapo, Chile, heard it emanating from a sandy hill, they dubbed the peak El Bramador, for its roars and bellows. Scientists today call it 'singing sand,' but they're all referring to the same thing: As sand grains shuffle down the slopes of certain sand dunes, they produce a deep, groaning hum that reverberates for miles.

But how these dunes produce this 'music' remains a much debated mystery. Another vexing question is why different dunes sing different tunes - and how can some even sing more than one note at a time?