An area called "Racetrack" in California's Death Valley gets its name from the large stones that seem to "race" around the barren dry lake bed. The rocks, some weighing more than 100 pounds, leave snaking grooves in the lake bed. Some of these grooves are thousands of feet long. Studies have been made of the area and measurements taken of the rocks and their positions. It appears that the rocks are actually moving, but no one has yet actually seen one of them doing so. More here.