Death Valley's "sailing stones" mystery has been solved

 People have been marveling over the "sailing stones" of Death Valley's Racetrack Playa for decades. The playa is a broad, flat stretch of sand which bakes in the harsh desert sunlight, and frequently gets down below freezing at night. 

No one had ever seen the large stones move, but the evidence was there: tracks in the dirt, as if the stones had been dragged, sometimes for several hundred feet. Years ago, scientists placed GPS trackers on the stones and verified that the stones were indeed moving. But no one knew when or why, until now.

Researchers at the University of San Diego spent two years waiting to catch the rocks in the act, so to speak. They set up rocks with GPS tags and a weather station, then waited. But as it happened, the researchers were there on site when the rocks started moving, and they were able to witness it with their own eyes.

It turns out that if enough rain falls on the playa late enough at night that it forms pools and then freezes, and this is followed up by a breeze first thing in the morning, the rocks start drifting on the ice. The morning sun warms the ice just enough to get a thin layer of water atop it, which is extremely slippery. Slippery enough that a gust of wind can push a very large stone across the desert floor.