California Has Sand Dunes That Hum When You Slide Down Them

Late on a calm afternoon in the Mojave Desert, a family reaches the top of a golden dune, out of breath and laughing, and sits down on the sharp crest to rest. Then they push off and slide down the steep side together, a wave of sand pouring ahead of them. A moment later the whole hill begins to hum: a deep, low note, like a far-off aeroplane, or a cello played somewhere under the floor.

This is the Kelso Dunes, in the Mojave National Preserve in California, and the sound is real. The US National Park Service describes it as a deep, eerie rumbling that you can feel in your bones. Only seven dune fields in North America are known to boom, and Kelso is one of them.

Why only some dunes sing

Most sand stays silent however hard you slide on it, so the singing dunes must have something special. According to the park service, the grains have to be smooth and much the same size, and the sand has to be dry. The sand at Kelso has travelled about 35 miles to get there, and the long journey has polished its grains until they are extremely round and smooth. The dunes themselves rise more than 600 feet, which gives the sand plenty of slope to pour down.

• over 600 ft height of the Kelso Dunes
• 35 miles how far their sand travelled to get there
• 7 dune fields in North America known to boom
• 70–105 Hz pitch of the hum measured at Eureka Dune

Around the world, booming sand has been known for centuries. In a 2010 review, Melany Hunt and Nathalie Vriend of the California Institute of Technology counted at least 40 sites where it happens. The sound has one strong low note with higher harmonics above it, and depending on the place and the season it can carry on for several minutes, even after the sand has stopped moving.

What people often get wrong

It is tempting to think the wind plays the dunes, like breath across the top of a bottle. It does not: a dune sings when its sand avalanches, which is why visitors set it off by sliding down the steep side. Nor is the hum simply the noise of countless grains rubbing together. The Caltech researchers explain it as sound resonating in a surface layer of the dune, so the hill itself behaves rather like an instrument that holds the note and makes it ring.

How scientists listened

To study it, the team spent about 25 summer field days on Eureka Dune in Death Valley National Park, a dune some 200 metres high, using geophones pressed into the sand to measure the vibrations running through it. In 2015 they reported in the journal Physics of Fluids that the dunes sing at between 70 and 105 hertz, and that the steady boom is a different thing from the short bursts that come before it. The bursts are waves rolling along the surface; the boom comes from waves travelling through the body of the dune and reflecting off layers inside it.

It is a lovely thought that a hill of sand can have something like a voice, tuned by its own shape and size. If you ever climb one, go on a dry day, gather some friends on the crest and slide down together. The desert may well sing back.