If you've ever been to Death Valley, you might have stopped by a place called Racetrack Playa. It's a dried-up lake famous for the rocks that have, for some mysterious reason, been moving around the lake bed, leaving trails in their wake. Some move further than others, some move in different directions than others, some change direction, but they move.
I bring this up because we now have an explanation as to why. The first thing to note is that just because Death Valley is notoriously hot doesn't mean it's always hot there all the time. Deserts, in fact, have wildly shifting temperatures. A key function of water is that it moderates temperatures; it takes longer than air to heat up and longer than air to cool back down. The abundance of it makes life possible not just because you can drink it, but also that it spreads out heat evenly enough that temperatures don't get too extreme.
Which means Death Valley, a desert, can get cold as well. But that doesn't mean it doesn't have any water. It does rain once in a while, and a thin layer of water does show up on the playa from time to time as a result. When the area gets cold enough at night, it changes into a very thin layer of ice... that the rocks are laying on top of. In the morning, the temperature heats up, the ice cracks, and when the wind picks up (it doesn't take much), the ice hydroplanes and pushes against the rocks, driving them down the playa and leaving tracks in the mud.
And nobody is visiting under those conditions.
Credit cousins Richard and Jim Norris, who've been on the case since 2011 and managed to finally crack the code last winter, on December 20, when for the first time they happened to be around to see it in person. Or, well, blame them maybe, if you're one of those people that really kind of hoped the mystery never got solved because it was more fun that way.