How do sand dunes form, move and sometimes sing?

Dunes form and move because wind keeps shifting sand grain by grain. Wind pushes sand up the gentle upwind side of a dune, where grains creep or bounce along in short hops. At the crest the sand tumbles down the steep downwind face, called the slip face. Each grain moved from front to back makes the dune creep forward. Dunes are most common in deserts because the lack of moisture hinders the plants that would otherwise hold sand in place.

The classic moving dune is the barchan, a crescent-shaped mound that forms where wind blows mainly from one direction and the sand supply is small. Barchans migrate with the wind, eroding on the windward side and building up on the leeward side, at anywhere from about one metre to 100 metres per year. A group of dunes in China's Ningxia Province moved more than 100 metres per year between 1954 and 1959. In the 1890s, the town of Eucla in Western Australia had to be moved because of dune drift.

Smaller barchans move faster than big ones, so they catch up with the back of a larger dune and appear to punch straight through it and come out the other side.

Some dunes also sing. About 35 desert locations around the world have dunes that produce a roaring, booming sound of up to 105 decibels, lasting minutes, when the sand slumps or avalanches after wind blows over the crest or someone walks near it. Scientists still debate exactly how.