Mystery of 'Sailing Stones' in Death Valley Solved: How These Rocks Roll (VIDEO)

A mystery has long plagued the Death Valley. Littered across the surface of the Racetrack Playa, a dry lake, are hundreds of rocks that seem to have been dragged across the ground, leaving synchronized trails that can stretch for hundreds of feet. Now, scientists have uncovered the mysterious force that may be moving these rocks.

These stones can sit for a decade or more without moving. In fact, they're stationary enough that the researchers didn't originally expect to see motion in person. That's why they monitored the rocks by installing a high-resolution weather station capable of measure gusts to one-second intervals. They also fitted 15 of the rocks with custom-built, motion-activated GPS units.

Then, in December 2013, they saw movement. The researchers came to the playa and found that it was covered with a pond of water about three inches deep. Shortly after, the rocks began to move.

"Science sometimes has an element of luck," said Richard Norris, one of the researchers, in a news release. "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."

For the rocks to move, a rare combination of events needs to occur. First, the playa needs to fill with water, which has to be deep enough to form floating ice during cold winter nights but shallow enough to expose the rocks. During the night, the pond freezes to form thin sheets of "windowpane" ice, which need to be thin enough to move freely but thick enough to maintain strength. When this ice melts, light winds drive the ice across the playa and push the rocks in front of them.

The findings upend previous theories that proposed that hurricane-force winds, dust devils, slick algal films, or thick ice sheets were the cause of this movement. Surprisingly, it's likely that others saw these rocks moving without even realizing it.

"It's possible that tourists have actually seen this happening without realizing it," said Jim Norris, one of the researchers. "It is really tough to gauge that a rock is in motion of all the rocks around it are also moving."

The findings are published in the journal PLOS One.

Want to learn more? Check out the video below, courtesy of YouTube.