#98: Roaming Rocks of Death Valley

Large boulders like this one wander across the flat clay surface of Racetrack Playa, a dry lake bed in Death Valley National Park in California, leaving long furrows but no hint of what propelled them. Last summer, NASA’s Cynthia Cheung may have discovered their secret: The rocks, some weighing several hundred pounds, probably glide on collars of ice that form around their base. When rain or snowmelt wets the valley, the collars act as flotation devices, Cheung says. The boulders then slide so easily that high winds can send them scooting, improbably and beautifully, across the slick surface.