On a cracked lakebed in Death Valley called Racetrack Playa, there are a collection of stones, some weighing several hundred pounds, that clearly, mysteriously, move. They leave long serpentine trails behind them in the dirt, and for decades visitors have guessed at how they migrate across the desert floor. Hurricane force winds, sliding sheets of ice — and, of course, aliens — were all contenders, but then last December the cousins Richard Norris and James Norris caught the rocks in the act.

In a paper published in Plos One, they describe watching a thin layer of ice break into large panes and get pushed by a light wind against the boulders, which then began to slide through the mud at about 15 feet a minute. "We were sitting on a mountainside and admiring the view when a light wind kicked up and the ice started cracking," Richard told the LA Times. "Suddenly, the whole process unfolded before our eyes."

Geologists have been studying the sailing stones since 1948, but despite years of research, no one could conclusively say how the rocks moved. That was partly due to the fact that they moved very rarely. Conditions need to be perfect. First there needs to be rain — obviously a rarity in Death Valley — then the temperature needs to drop below freezing, then it has to warm up fast enough to rapidly melt the ice, and then there needs to be wind to break the ice and push it against the stones. Previous attempts at setting up time lapse cameras never caught the rocks in motion.

In 2011, the Norrises set up a weather station nearby and affixed GPS devices to 15 rocks. At the time, they ascribed to the hurricane-force wind theory, but in November 2013, a freak storm dumped an inch and a half of rain, which then froze. When they visited the Valley a month later, they saw a light wind crack the ice into large, thin panes and plough the boulders through the mud.



From the archives: Big Science in New Mexico: Prometheus in the desert