For decades, scientists and visitors noticed that the rocks at Death Valley's Racetrack Playa seemed to scoot through the mud on their own, leaving tell-tale tracks in the dirt. No one had ever seen them move, though. Was it animals? Earthquakes? Gravity? Aliens?
In 2011, two cousins from California, one an engineer, one an oceanographer, decided to find out once and for all. They set up timelapse cameras and a weather station, and tagged some of the rocks with GPS trackers. Then they sat back, for what could have been a very long wait--the rocks don't move very often.
In December 2013, though, the cousins got lucky. They headed to the playa to check on the instruments, and found about a third of the valley covered in an ice-encrusted pond. As the sun heated up the day, the ice cracked into pieces. Then the wind began to move the ice sheets. And the ice sheet, in turn, started nudging the rocks. They came back later and were able to record a video of the phenomenon. And by the end of the winter, the most active rock had moved 735 feet.
See their video of the moving rocks: