for National Geographic News
It sounds like a low-flying propeller plane or maybe the deep humming of an electric wire. The booming sound made by some sand dunes has been a mystery for centuries.
The sound is produced when sand on the surface of a dune avalanches. Scientists have long believed the friction between grains creates the strange noise.
But Melany Hunt, a mechanical-engineering professor at the California Institute of Technology in Pasadena, thinks there is something else at play too.
For the past three years she has been taking her students out to the desert of eastern California and Nevada. By sliding down the dunes on their bottomsliterally doing science by the seat of the pantsHunt and her researchers set the sand in motion to measure the sound waves it makes.
They have found that the sound continues even after the movement has stopped. Also, the sound that a sand dune makes in winter differs from the sound it makes in summer.
Hunt believes that when the sand on the surface is disturbed, the friction between sand grains creates a noise that reverberates back and forth between dry sand on the surface and wet sand below.
"That may be why smaller dunes don't make sound," Hunt explained. "They haven't been around long enough to form that hard layer of [wet] sand."
Records of the sound are centuries old. Hunt has a book, Tales of Travel, published in 1923, which mentions that explorer Marco Polo knew about it. The tribes of the Sahara in Africa are said to have thought God was speaking to them through the sand.
Hunt became interested in the phenomenon when a colleague played her recordings of the sound he had made 25 years ago.
Then a group of students went out to the Kelso Dunes near California's Mojave Desert. They brought back a jar of the booming sand. Shaking the jar, they found, produced a burping sound. This did not happen with sand taken from the beach, for example.
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