Since the 1940s, people have been scratching their heads about the apparently strange goings-on in a dry lakebed in Death Valley called the Racetrack Playa. There, every 10 years or so, stones as big as 700 pounds (318 kilograms) mysteriously seem to move around on their own, leaving long tracks behind them in the parched desert surface [source: Starr]. Over the years, various explanations-- from dust devils to films of slippery algae -- have been proposed, but none of them seemed too convincing.
Finally, though, in 2011, researchers from Scripps Institution of Oceanography at the University of California-San Diego decided to solve the enigma. Since the National Parks Service wouldn't allow them to attach GPS devices to the rocks themselves, they brought in 15 similarly sized pieces of stone and monitored them. It took two years, but they finally got the answer. In wintertime, the playa sometimes fills up with a thin layer of water from rainfall, which freezes overnight and forms thin sheets of ice. When the sun comes out the next day, the ice melts and cracks into panels that light winds then blow across the ice—carrying the rocks with them. But the stones typically slide at a speed of only a few inches per second, slowly enough that visitors can't really see the movement from a distance [source: Starr].