Since the 1940s, researchers have wondered what has moved hundreds of rocks — some weighing as much as  700 pounds — across the ground at Death Valley’s Racetrack Playa. These so-called “sailing stones” leave synchronized trails that can stretch for great distances.

Now, researchers have made first-hand observations of the phenomenon.

Although the researchers did not originally anticipate that they would see the process in person, an experiment that was set up in winter 2011 paid off in December 2013 when several of the researchers went to Death Valley and found that the playa was blanketed with a pond of water several inches deep. Soon after, the rocks started moving.

“Science sometimes has an element of luck,” said Richard Norris, a paleobiologist at the Scripps Institution of Oceanography. “We expected to wait five or ten years without anything moving, but only two years into the project, we just happened to be there at the right time to see it happen in person.”

Their experiment reveals that moving the rocks necessitates several events. First, the playa fills with water. As nighttime temperatures drop, the pond freezes to form thin sheets of “windowpane” ice. On sunny days, the ice start to melt and break up into big floating panels, which light winds move across the playa, pushing rocks in front of them.

Previous theories about the cause of the sailing stones included extremely strong winds, dust devils and slick algal films among others.

“The last suspected movement was in 2006, and so rocks may move only about one millionth of the time,” said Ralph Lorenz of the Applied Physics Laboratory at the Johns Hopkins University. “There is also evidence that the frequency of rock movement, which seems to require cold nights to form ice, may have declined since the 1970s due to climate change.”

The study’s findings are described in the journal PLOS ONE.