Rocks that mysteriously slide across a dry lake bed are thought to be nudged along by large ice sheets. But previously, no one could fully explain how the rocks — some weighing more than 300 kilograms — scoot across Racetrack Playa in California.
Now a team led by Richard Norris at the Scripps Institution of Oceanography in La Jolla, California, has caught the rocks in action after tagging them with Global Positioning Satellite markers. On 20 December 2013, pools of ice in the lake started cracking in the morning sun. A gentle breeze then bumped 'windowpane' ice sheets against the rocks, moving them at a rate of 2–5 metres per minute. When the ice melted, more than 60 rocks had budged, leaving freshly formed trails (pictured) behind them. By the end of the winter, the farthest-moving rock had travelled 224 metres.