For decades, heavy rocks in the desert seemed to move by themselves, leaving long trails behind them like invisible giants were dragging them across the sand!
In a dry lakebed called Racetrack Playa in Death Valley, California, there is a mystery that puzzled scientists for nearly 100 years. Heavy rocks, some weighing hundreds of pounds, move across the perfectly flat ground, leaving long, winding tracks behind them. But nobody ever saw them move - until recently.
For a long time, people had wild theories. Some thought it was aliens, others thought it was magnetic fields or pranksters. The tracks were clear, but the rocks seemed to stay perfectly still whenever humans were watching.
In 2014, scientists finally caught the rocks "in the act" using GPS and cameras. It turns out the answer is a rare combination of ice, water, and wind:
Even though the ice is thin, it acts like a giant sail. Because the ground is so muddy and slippery, the ice provides enough force to slowly slide the heavy rocks across the desert at a few inches per second.
The "walking" rocks of Death Valley are moved by thin sheets of ice on rare windy, rainy days. It's a perfect "natural machine" that turns a tiny breeze into enough power to move a heavy boulder.
