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If you head out to the deserts of Morocco you might hear some haunting music. It's not made by humans, it comes from the sand dunes. Why they make the sounds has long been a mystery. Even the thirteenth century explorer Marco Polo wrote about the eerie noise. But now a French scientist called Stephane Douady has come up with an answer. Image: Stephane Douady slipping down a sand dune Credit: Mathias Théry and Etienne Chaillou