Mystery of Death Valley's 'Sailing Stones' Solved

— -- The mystery of the sailing stones has been solved.

For years, enormous stones have been moving across the Racetrack Playa of Death Valley National Park, leaving engraved trails in the muddy surface behind. No one understood how, though there was plenty of speculation, according to researchers at the Scripps Institution of Oceanography at University of California San Diego.

Last year the researchers, Richard Norris and James Norris, placed stones equipped with GPS devices on the same stretch of land, then waited and watched.

"We recorded the first direct scientific observation of rock movements using GPS-instrumented rocks and photography, in conjunction with a weather station and time-lapse cameras," the authors wrote in a study published in the journal PLOS One.

On Dec. 20, 2013, they witnessed 60 rocks move across the land.

They discovered that the very thin ice that trapped the rocks during winter melted in the midday sun, and then the ice and water and rocks were all blown by wind, making it seem as if the stones moved by themselves.

"In contrast with previous hypotheses of powerful winds or thick ice floating rocks off the playa surface, the process of rock movement that we have observed occurs when the thin, three to six [millimeter], “windowpane” ice sheet covering the playa pool begins to melt in late morning sun and breaks up under light winds of (about four to five meters per second)," they wrote.