The Racetrack with one of its large rock minions crossing its surface. This one actually moved to my camera so that it could be photographed. Just kidding.

There are perhaps several hundred of these rocks with trails on the Racetrack playa. The force that allows movement is still up for speculation but the most obvious culprit would be a combination of water (or more likely thin sheets of ice) and wind. These rocks only seem to move once every few years and after 4 to 8 years their old paths have been replaced by new ones. No one has ever seen these rocks move. I suspect that a lot of the reason why is because the playa would require a wet surface and that same weather condition would make the dirt road to the playa impassable hence no one would be on the scene to see them. Rocks can double back on themselves, do circles and leave zig-zag patterns and travel uphill. I have seen a few interesting scientific papers on the subject floating around on the Internet.