Scientists recently solved a longtime mystery involving singing sand dunes.

There are about 35 deserts and several beaches around the world with sand dunes that can actually make haunting, melodious tunes, and a few years ago researchers discovered that the sounds came from falling sand. But what remained a mystery until now is why some dunes could croon in more than one pitch at the same time. To find out, scientists compared two vocal dunes, one in Morocco and one in Oman. The Moroccan dunes are one-note wonders, known to sing only in the neighborhood of G-sharp. The Oman dunes are more versatile, with a cacophonous nine-note range and the ability to hit multiple notes simultaneously, creating eerie harmonies.

So why do some dunes sing a different tune? The answer was in the sand. Specifically, sand grain size. The Moroccan sands were pretty uniform, while the sands of Oman where varied. Varied sizes made for more interesting music. You can hear the differences in the video below.

Incidentally, if you happen to visit a singing dune that’s having a quiet moment, you can start the symphony yourself just by sending some sand down the slope. Check out some techniques for playing dune music below.

Photo via Wikicommons; videos via American Geophysical Union and Stephane Douady