For years scientists have theorised about how large rocks, weighing up to hundreds of kilograms, zigzag across an area of California's Death Valley National Park, leaving trails.
Now, researchers at the Scripps Institution of Oceanography at the University of California have photographed "sailing rocks" being blown by light winds across the former lake bed.
Cousins Richard Norris and James Norris said the movement was made possible when ice sheets that formed after rare overnight rains melted in the rising sun, making the hard ground muddy and slick.
When the ice sheets broke apart, winds caused them to bunch up against the stones and the ice acted as a sail, pushing the rocks across the lake bed.
On one December day the cousins catalogued 60 rocks moving across the pancake-flat surface of Racetrack Playa.
"Observed rock movement occurred on sunny, clear days, following nights of sub-freezing temperatures," they wrote in a report published in the online scientific journal Plos One.
The conclusion proves theories that have been floated since geologists began studying the moving rocks in the 1940s.
Richard Norris, 55, a palaeobiologist at Scripps, and James, 59, a research engineer, launched their "Slithering Stones Research Initiative" in 2011, the Los Angeles Times reported. They installed a weather station in the area and placed 15 stones equipped with global positioning devices.
At the end of last year, the Norrises returned to inspect the instruments.
"We found the playa covered with ice," Richard recalled. "We also noticed fresh rock trails near shards of thin ice stacked up along the shoreline."
The following afternoon, "we were sitting on a mountainside and admiring the view when a light wind kicked up and the ice started cracking," he said. "Suddenly, the whole process unfolded before our eyes."