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?

A trio of Parisian biophysicists think they know the answer. It's not necessarily the motion of the sandy ocean that determines the pitch of the note—it's the size of the grains, though why the size matters is still unknown.

The researchers first tracked down a pair of singing dunes, one in Morocco, the other in Oman. Working literally by the seat of their pants, they scooted feet-first down the hills to trigger the avalanches. They found that, while the Moroccan hill moaned at a steady 105