A playa (at least outside of the young urban vernacular) is a dry lake bed. The 2.5-mile "Racetrack Playa" on the west side of Death Valley National Park, in California, is famous for its mysterious sliding rocks.
In the 100 years since these large (about a foot tall and as much as several hundred pounds) sliding rocks were first noticed, apparently nobody has actually ever seen them move. They see only the long trails marked out in the cracked surface of the lake bed.
So why do they move? It's not gravity: The playa is extremely flat. Geologists' strongest hypothesis points to rain. Though the region is extremely dry, when it does rain, the silt and clay surface turns into a muddy Slip-N-Slide. Then, a strong gust of wind—up to 70 mph—can get the rock moving, and once in motion, slower winds can keep it going. The rocks change direction, the theory goes, with the wind.
(Hat tip to Alex B., who passed along this article from Geology.com)