Racetrack Playa: The Mystery of Death Valley's Moving Rocks, California

In a remote valley in the north of Death Valley National Park lies Racetrack Playa, a dry lakebed almost perfectly flat and cracked into tiles. Scattered across it are rocks, some small, some reportedly as heavy as 700 pounds, and behind many of them are long, shallow tracks carved into the mud. Some trails run straight, others curve or zigzag. The rocks seemed to have moved on their own. The rocks themselves came from the surrounding mountains. They eroded off the slopes and tumbled down onto the playa. How they moved across it was one of the desert's longest-running mysteries. Theories over the decades included powerful winds, slippery algae, thick ice and even magnetism. For years, no one had ever seen a rock move. In 2011, a team led by Scripps Institution of Oceanography paleobiologist Richard Norris, with his cousin James Norris, set out to watch. They set up a weather station near the playa, along with time-lapse cameras and GPS-equipped rocks. Their chance came in the winter of 2013–2014. Rain and snow had left a shallow pond covering part of the playa, no more than about 7 centimeters deep. On cold nights, it froze into thin sheets of "windowpane" ice, less than 5 millimeters thick. On December 21, 2013, just before noon, the ice began to break up, with popping and cracking sounds coming from all across the frozen pond. As light winds of about 3 to 5 meters per second pushed the floating ice panels, the panels shoved the rocks ahead of them. The rocks slid slowly through the mud beneath the water, a few meters per minute, carving fresh trails. Over that winter, the team saw it happen several times. In one event, more than 60 rocks moved, and some traveled more than 200 meters over multiple events. The findings were published in the journal PLOS ONE on August 27, 2014. Instead of hurricane-force winds or thick ice lifting the rocks, it turned out to be thin ice and gentle breezes. The conditions are rare, and the largest rocks weren't seen moving. Researchers have also suggested that rock movement has become less frequent since the 1970s, possibly because of a changing climate. But the trails remain on the playa, recording each time the conditions came together.