Thrum, thrum, thrum!

My tracks down a slip face at the Kelso Dunes, in the Mojave National Preserve, California. This is a "singing" (or "booming") dune--as you slide down the face, you hear a deep tone from the sand. It's astonishingly musical for such improbable materials, like a low note on a cello. To "sing" evidently the dune sand has to be very dry and well sorted--i.e., all the grains about the same shape and size. (It won't sing after a thunderstorm!) The phenomenon is more than a little eerie, especially when you're out there by yourself. You can see why the stories in the _Arabian Nights_ and so on attributed the singing to djinns or other spirits!