Walking rocks in the desert leave tracks

The massive rocks of Racetrack Playa move because of window-pane thin sheets of ice, not gravity or heavy winds. Many of these stones weigh over 700 pounds, yet they leave trails hundreds of feet long in the dry mud.

For nearly a century, no one saw them move. Theories ranged from magnetic fields to alien intervention. The truth was finally captured using GPS sensors and time-lapse cameras during a rare weather event.

When a few inches of rain flood the playa and freeze at night, they form sheets of ice as thin as glass. When the sun rises, the ice breaks into large floating panels.

Even a light breeze provides enough shove against these ice sheets to move the heavy boulders. The rocks don't slide on the mud; they are bulldozed by the ice.

The ice panels can be hundreds of feet wide but only millimeters thick, acting like a giant sail. This creates enough momentum to drag a rock at the speed of about 15 feet per minute.

Once the water evaporates and the ice melts, only the rock and its deep furrow remain in the cracked earth. Paradoxically, the rocks move most efficiently on the coldest, clearest days in one of the hottest places on the planet.