Some sand dunes make a deep, resonant hum when sand slides down their sides. Out of the 30 so-called "singing dunes" on earth, four of them are in California. They're a bit of a mystery, but one that’s closer to being cracked.

Anything that makes the sand move can induce their song: wind or just walking on them. Sonoma County-based bio-acoustician Bernie Krause has been recording singing dunes in California and Nevada for thirty years.

"The singing dunes occur in the literature from T.E. Lawrence all the way through to Native American myths from the Southwest," he said, "They heard these sounds that would come booming across the desert, and they weren't sure where they came from." 

Scientists are just now starting to crack the mystery of the melodies.

By reproducing the sounds in a lab, a French scientist concluded that the size of the sand grains explains why different dunes hum different tones, and why some dunes can produce more than one note at once. Simon Dagois-Bohy of Paris Diderot University published his results in the journal Geophysical Research Letters.

Scientists still disagree over how exactly the singing works.

Read more:

• More Clues About Singing Sand: KQED Quest community blogger Andrew Alden digs into the disagreement -- and adds some cool videos.
• Wild Sanctuary: Bernie Krause's website.
• Booming Sands: A video from NOVA on a CalTech team's research on singing dunes.