For decades the heavy rocks that carve trails across a Death Valley lakebed were never seen moving, until cameras caught them gliding on thin rafts of melting ice pushed by a light wind

On Racetrack Playa, stones heavy enough to need two hands leave grooves in the dried mud as if they walk. A weather station, fifteen GPS-fitted rocks and a set of time-lapse cameras finally recorded the cause, and it was almost nothing: a few millimeters of windowpane ice and a breeze. On a flat dry lakebed in Death Valley National Park, heavy rocks sit at the end of long grooves they have plowed across the mud. The trails run for tens of meters, some bending in sharp turns or doubling back, yet no one had ever watched a rock actua... [6704 chars]