The sailing stones of the Racetrack Playa, a dry lakebed in Death Valley, have been the subject of a mystery since the 1940s. The playa is dotted with stones, some as large as 700 pounds (320kg), with long tracks behind them, as though they have been performing a synchronised dance.
Although there have been many theories about how the rocks might be moving on their own -- including dust devils, hurricane-force winds, films of slippery algae or thick sheets of ice -- none had ever been confirmed, nor had any human seen the rocks actually moving.
Until now, that is. A team of researchers from the Scripps Institution of Oceanography at UC San Diego decided they were going to solve the mystery once and for all. In the winter of 2011, they brought in a high-resolution weather station to measure wind at one-second intervals, and brought in 15 rocks fitted with GPS devices (since the National Parks Service would not allow them to use the native rocks).
Since the stones move so rarely, though -- maybe once every 10 years -- they were prepared to settle in for a long wait. However, they had a stroke of luck. Study co-authors Jim Norris and Richard Norris visited the site in December 2013 to discover the playa covered in water 3in (7cm) deep.
"On December 21, 2013, ice breakup happened just around noon, with popping and cracking sounds coming from all over the frozen pond surface," said Richard Norris. "I said to Jim, 'This is it!'"
As it turns out, the movement requires the perfect concatenation of events. First, the playa has to fill with water, which must be deep enough to form floating ice during winter, but still shallow enough that the rocks are exposed.
When the temperature drops at night, this pond freezes into thin sheets of "windowpane" ice, which then must be thick enough to maintain strength, but thin enough to move freely.
Finally, when the sun comes out, the ice melts and cracks into floating panels; these are blown across the playa by light winds, propelling the rocks in front of them.
And it's surprisingly gentle: the sheets of ice are only 0.25in (3-5mm) thick, moving under winds of 10mph (3-5 metres per second), pushing the rocks along at a speed of only a few inches per second -- a speed which is almost imperceptible at a distance unless you know what to look for.
"It's possible that tourists have actually seen this happening without realising it," Jim Norris said. "It is really tough to gauge that a rock is in motion if all the rocks around it are also moving."
But for all those mystery lovers out there, there may still be one to be solved.
"We documented five movement events in the two and a half months the pond existed and some involved hundreds of rocks," Richard Norris said, "So we have seen that even in Death Valley, famous for its heat, floating ice is a powerful force in rock motion. But we have not seen the really big boys move out there... Does that work the same way?"
The full paper, "Sliding Rocks on Racetrack Playa, Death Valley National Park: First Observation of Rocks in Motion", can be read online in the journal PLOS One.