For at least a century, the "slithering stones" of Death Valley—rocks, some of them boulders weighing more than 600 pounds, that move across the desert floor and leave long trails behind them—have presented a puzzle, and researchers have been trying to solve it since 1948. Explanations including dust devils, flooding, hurricane-strength winds, and even UFOs have been offered, but the mystery was finally solved when researchers caught the rocks in the act, National Geographic reports. Scientist Richard Norris and his cousin James Norris, an engineer, put GPS trackers on the stones, surrounded them with cameras, and were amazed to discover that the movement was caused by ice—in one of the hottest and driest spots on the planet.
The pair discovered that on rare occasions when conditions are just right—with rain on the usually dried-up lake bed called the "Racetrack Playa," followed by sunshine and wind—plates of thin ice push the rocks along the muddy desert floor, sometimes as quickly as several feet a minute, reports NPR. Richard Norris tells the Los Angeles Times that they found the playa covered with ice when they went to inspect the instruments last December. The next day, "we were sitting on a mountainside and admiring the view when a light wind kicked up and the ice started cracking," and "suddenly, the whole process unfolded before our eyes," he says. "There was a side of me that was wistful because the mystery was no more," James says. (Another recently solved mystery: how to classify the bizarre fossil of Hallucigenia.)