Some dunes sing

Certain dry sand dunes emit a loud, sustained low note when sand avalanches down their slip face.

Travellers from Marco Polo onward reported booming sands. The modern measurements find a pure tone, often between about 70 and 110 hertz, loud enough to hear a kilometre away, lasting as long as the avalanche lasts.

The grains are dry, rounded, and of a narrow size. As they shear, they synchronise. Whether the resonator is a surface wave or the bulk of the dune was argued; both schools agree the sound is the sand moving, not wind in a cavity and not folklore.