Imagine standing alone in the vast, searing emptiness of a desert. Suddenly, a sound begins. It is a deep, powerful hum that vibrates through the soles of your feet, a ghostly drone that rises from the very ground you stand on. It sounds like a colossal cello playing a single, haunting note or a phantom choir chanting from beneath the earth.
This is no trick of the wind, and you are not imagining it. You are hearing one of the planet’s most bizarre and extraordinary marvels: the song of a singing sand dune. Believe it or not, in dozens of locations across the globe, mountains of sand are alive with music.
An Unearthly Orchestra
For centuries, travellers whispered tales of these musical mountains. The great explorer Marco Polo encountered their eerie drone in the deserts of China and believed it to be the sound of evil spirits. In the 1800s, Charles Darwin recorded stories of a Chilean sand hill called “El Bramador,” The Bellower, that roared with an unearthly noise. For a long time, no one could explain it.
The sound only appears when the sand moves. A gust of wind can send a river of sand cascading down the dune’s steep face, awakening a sudden, booming chorus. Adventurous souls can even create the music themselves by sliding down the slope, conducting their own personal, geological orchestra. From the Mojave Desert in America to the dunes of Dunhuang in China, people have reported feeling the low-frequency rumble in their chest, a truly sensational experience that defies easy explanation.
Solving a Sonic Mystery
For years, scientists thought the sound came from vibrations deep within the dunes. They were wrong. The stunning truth is that the avalanche of sand itself sings. In 2012, a team of physicists from Paris Diderot University, led by Simon Dagois-Bohy, decided to unravel this sonic mystery once and for all. They travelled to two very different singing dunes to listen to their strange music.
Their first stop was a dune near Tarfaya in Morocco. No matter where they listened, this dune performed like a trained musician, producing a clean, steady note at about 105 hertz, a G-sharp. Their second stop was near Al-Askharah in Oman. The Omani dune was a wilder beast. It unleashed a chaotic blast of sound, a noisy jumble of almost every frequency from 90 to 150 hertz. The scientists had to find out why one dune sang with such discipline while the other screamed.
A Song of a Million Grains
The secret, they discovered, was hiding in plain sight: it was the size of the sand grains. When they examined the sand under a microscope, they found the Moroccan grains were all almost identical, measuring between 150 and 170 microns. The chaotic Omani sand, however, was a jumbled mess of different sizes, ranging from 150 all the way to 310 microns.
To prove their theory, the team took a sample of the noisy Omani sand back to their laboratory. First, they let the mixed-size sand slide down a ramp, and just as it did in the desert, it created a noisy, indistinct sound. Next, they used a fine sieve to separate the grains, isolating only those between 200 and 250 microns. When they sent this uniform, sifted sand down the same ramp, the result was astonishing. The sand sang a pure, clear note, just like the Moroccan dune.
The researchers proposed that when sand grains are all the same size, they flow like a perfectly coordinated army, sliding over one another at a consistent speed. Their tiny vibrations synchronise, merging into one powerful, coherent sound wave that we hear as music. In the messy Omani sand, the different-sized grains moved at different speeds, creating a disordered vibration that sounded like noise.
While this explains the different songs, one great mystery still remains. Scientists still do not fully understand the strange force that compels millions of individual grains to synchronise their vibrations so perfectly in the first place. The desert, it seems, has not yet revealed all its secrets.
