For decades, Death Valley held a mystery that scientists could not solve.

Large rocks were slithering across the valley floor, traveling hundreds of meters, leaving long tracks behind them.

But no one had seen them do it or could figure out how it happened.

So, scientists fitted rocks with GPS trackers and set up remote cameras to try to capture the rocks in motion.

Because the movements could be separated by decades, they expected this to be “the most boring experiment ever.”

But just two years in, they got very lucky: they were in Death Valley one cold December to change the GPS batteries … and saw the rocks move in person.

And figured out the rare combination of conditions that made it happen.

The valley floor had to cover with water, from rain or melting snow. The night had to be cold enough to form a thin sheet of ice across its surface.

The next day had to be warm enough to break up the ice into large floating panels, up to 50 feet wide.

Then a light, steady wind was needed to gently push the ice islands across the water, slowly nudging the rocks along with them, across the slippery mud bottom.

By midday, the ice and water had evaporated, and the scientists saw long trails behind the now displaced rocks. Their cameras and GPS trackers captured it all.