After centuries of speculation, French physicists have discovered the root cause of the peculiarly melodious roar that emanates from desert sand dunes. Marco Polo described them as the call of desert spirits who "at times fill the air with the sounds of all kinds of musical instruments."
Stéphane Douady of the Ecole Normale Supérieure in Paris was studying crescent dunes in Morocco when a colleague, Bruno Andreotti, accidentally triggered an avalanche, which was followed by an eerie rumble. The researchers carted 158 pounds of sand back to France and placed it in a doughnut-shaped stirrer to replicate the event. When sand grains move at random, the experiments showed, expanding and contracting airholes between them create sounds that cancel each other out. But if a large number of grains flow in unison, they generate an oddly musical boom. "All musical instruments have two parts, one of which excites the other to resonate. This is different because there is no excitation imposed from outside; it all comes from the synchronous motion of the grains. We still don't understand why they synchronize," Douady says.