The Sliding Rocks of Racetrack Playa

" One of the most interesting mysteries of Death Valley National Park is the sliding rocks at Racetrack Playa (a playa is a dry lake bed). These rocks can be found on the floor of the playa with long trails behind them. Somehow these rocks slide across the playa, cutting a furrow in the sediment as they move. "

http://geology.com/articles/racetrack-playa-sliding-rocks.shtml