Spooky rocks that move by themselves

"Inside Death Valley National Park in Racetrack Playa you can find these interesting rocks that apparently move by themselves. They're on the floor of the playa with long trails behind them.
Somehow these rocks move across the playa, cutting a furrow in the sediment as they move. No one knows or can give a real explanation, and no one has ever seen them in motion. Some of these rocks are really heavy, so how do they move?"

[via funbee]