It might just be the world’s slowest game of Red Light, Green Light: Huge rocks in Death Valley National Park race across the mud flats—but only when nobody is looking. The sailing stones, as they’re called, mystified park visitors and scientists for decades. But one man believes he’s solved the puzzle.

The dry lakebed known as the Racetrack playa looks much like the rest of Death Valley—cracked and dry in the summer and frozen in winter. But nowhere else in the park will you find strange tracks trailing behind each big rock and boulder. It looks as though the stones have dragged themselves through the desert, or as though they have been dragged, by some great cosmic hand. 

All summer, the stones are still. Through autumn, they don’t budge. Then winter roars in and creeps out. By spring, the stones have moved again.

Natural and supernatural theories have abounded: Unsurprisingly, many attributed the rocks’ stealthy movements to aliens. Some said wind was the culprit; others ice; others rain; still others, mystical energy fields. Some people even steal rocks from the park, hoping to harness their magical powers. 

Scientists have set up experiments in the playa since the 1940s, trying to understand what makes the sailing stones sail. But all the results have been inconclusive, and despite frequent checks, nobody has ever been able to catch the rocks in motion.

It took a space researcher to crack the case. Planetary scientist Ralph Lorenz was working with NASA, setting up miniature weather stations in Death Valley, when he first became interested in the stones. (Conditions in the park are so severe that it’s often used as an experimental stand-in for Mars.) Although his original work was focused on summer in the desert, Lorenz realized his instruments would work just as well for monitoring the playa’s rocks in the winter. Lorenz and his team pored over images of the stones’ trails, looking for some clue.

One rock stood out. “We saw one instance where there was a rock trail and it looked like it hit another rock and bounced, but the trail didn't go all the way up to the other rock, like it was repelled somehow,” Lorenz told Smithsonian.com. “We thought if there was a collar of ice around the rock, then it might be easy to imagine why it might bounce.” 

The final breakthrough came not in the laboratory or even in the desert, but in the kitchen. Lorenz poured a little water in a plastic container, then dropped in a small rock and put the whole thing in the freezer. When he took it out, he had a rock half-sheathed in ice. That rock went into another dish, this one filled with water atop a layer of sand. He set the rock in the water, ice side up, and blew on it to give it a gentle push. The rock floated across the water, scraping a path through the sandy bottom as it went. Lorenz had found the answer. 

“Basically, a slab of ice forms around a rock, and the liquid level changes so that the rock gets floated out of the mud,” he said. “It’s a small floating ice sheet which happens to have a keel facing down that can dig a trail in the soft mud.” 

Lorenz published his findings in the American Journal of Physics in 2011. Once he knew what to look for, he was even able to catch the rocks in motion.

Not everybody is satisfied with the ice-raft explanation. Park visitors ask why it happens, but they don’t want to hear about science, park ranger Alan Van Valkenburg told Smithsonian.com. “People like a mystery—they like an unanswered question.”