In the southwestern United States and approximately 40 locations around the world, large sand dunes can generate a loud booming sound during a natural or induced avalanche. The sound builds over time to a single frequency varying from 75 to 105 Hz plus harmonics depending on the dune location and time of the year. This talk will outline the historical references to this phenomenon as well as recent field work at Caltech involving seismic refraction, ground-penetrating radar, and sand sampling. In addition, the talk will describe the reported explanations of this natural mystery.