Racetrack Playa, Calif. Even NASA cannot explain it. It's best to gaze in wonder at the sliding rocks on this dry lake bed in Death Valley National Park. Racetrack Playa is almost completely flat, four kilometres from north to south and two kilometres from east to west, and covered with cracked mud. The rocks, some weighing hundreds of pounds, slide across the sediment, leaving furrows in their wakes, but no one has actually witnessed it. Is it the wind?