Heavy rocks seems to move by themselves in one part of California's Death Valley, their means of locomotion a scientific mystery, LiveScience reports. Though no one has ever seen them move, rocks the size of large soda bottles criss-cross the surface of Racetrack Playa, leaving trails. "You don't expect 20-pound rocks to go sliding across the ground very easily, but they seem to do that on occasion," says a NASA scientist.
Scientists don't think it's pranksters (or aliens). But they have a promising theory: Temperatures in the playa—a dried out lake-bed—can drop below freezing in the winter months and allow ice to form. So maybe the rocks are sliding around on a frozen surface? "It's pretty clear these rocks are assisted by ice somehow," says the scientist. A new paper will be out this year on the subject.