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One of several hundred rocks that have left trails as they moved across the surface of Racetrack Playa in Death Valley National Park. (Louis Sahagun / Los Angeles Times)

DEATH VALLEY NATIONAL PARK, Calif. — Two researchers just happened to be in the right place at the right time to solve a mystery that has perplexed geologists for decades.

Richard and James Norris were at Racetrack Playa on Dec. 31 last year when they heard ferocious cracking sounds as the frozen lake began breaking apart under sunny skies. Huge panes of ice were lifted by light winds and pushed against rocks, some weighing 200 lbs. They sheets of ice became sails and the rocks began to move across the slick mucky bottom of the normally dry lake bed.

James Norris grabbed his camera and the photos he snapped that day provided the evidence that explained what moves rocks across Racetrack Playa, The Los Angeles Times reported .

Theories over the decades have included sporadic hurricane-force winds when the surface is covered with rain water, or rocks carried across the mud by small rafts of ice, or even UFOs.

But until the Norrises had an incredible stroke of luck that day in December, no one had witnessed and recorded the phenomenon. The findings were formally presented Wednesday in the online scientific journal PLOS ONE.