(This video is no longer available for streaming.) At roughly 30 locations around the world, sand dunes seem to sing, producing haunting and baffling sounds. Marco Polo noticed the phenomenon in the Gobi Desert. Ancient travelers have heard it in the Sahara. Even Charles Darwin puzzled over it in the Chilean desert. For centuries, no one was able to explain why it happens. Now, engineers Melany Hunt and Christopher Brennen of the California Institute of Technology are on the case. Their theory is that the booming sand dunes act like enormous musical instruments.