Death Valley, California, USA. Rocks moving across the playa and leaving a trail. The mechanisms are still controversial, but it seems to occur when there are strong winter winds while the playa is wet and extremely slippery.

It isn't restricted to Racetrack Playa--I've seen similar trails left by things like abandoned tires on other playas. The unusual thing about Racetrack seems to be its being adjacent to cliffy outcrops that can drop rocks on the playa! Most playas are out in the flats, remote from any source of objects that could fall on them to be pushed.

UPDATE: Just in the last couple of years, the stones have finally been seen to move. (Some of them were wired for GPS!) It turns out thin ice sheets are acting as sails:
www.nature.com/news/wandering-stones-of-death-valley-explained-1.15773
As I noted above, I've seen similar trails on other playas, involving things like old tires, so it's not just the Racetrack!