The Racetrack Playa must first fill with water — deep enough to float massive sheets of ice, yet still shallow enough to leave the rocks exposed. Nighttime temperatures must then get cold enough to freeze the water, forming mammoth-size ice panes — thin enough to glide across the lake bed, yet thick enough to gain momentum and clear the stones in their way.

As the next day’s afternoon sun thaws the ice, these sheets break apart into chunks that, with any luck, a steady wind will propel across the playa pool. When the ice chunks hit the rocks — ranging from pebble to boulder-size pieces weighing up to 200 pounds — the stones are driven across the soft mud below, leaving behind their signature trails.

The findings explaining the process were published on Wednesday in the online science journal PLOS ONE

“I’m amazed by the irony of it all,” James Norris, a research engineer, told the Los Angeles Times. “In a place where rainfall averages two inches a year, rocks are being shoved around by mechanisms typically seen in arctic climes.” He added that “the movement is incredibly slow. These rocks clock in at about 15 feet per minute.”

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Richard Norris, 55, a paleobiologist at the Scripps Institution of Oceanography, and James Norris, 59, are cousins and researchers on the study. The men embarked on their mission to solve the mystery in 2011.

With some help, they set up a weather station in the park. They inserted global positioning devices into rocks and planted them at the southern end of the playa where the stones begin their adventure. Then they waited.

“As one of the other researchers on this project said, ‘Probably the most boring experiment ever,'” James Norris said in a video about the experiment, according to Discovery Magazine.

Until it wasn’t.

A week before Christmas in 2013, the cousins returned to Death Valley to check on their rocks. That’s when they found all the elements needed to explain the phenomenon: a playa covered with ice, some afternoon sunshine and a light, steady wind.

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“Suddenly, the whole process unfolded before our eyes,” Richard Norris told the L.A. Times.

They heard the ice crack. They saw the rocks begin to sail. James Norris grabbed a camera — and their photographs became the final piece needed to help solve the decades-long puzzle.

“The largest observed rock movement,” they wrote in their study, involved more than 60 rocks on Dec. 20, 2013, some of which moved up to 224 meters, or more than 700 feet, between December 2013 and January 2014 in “multiple move events.”