Desert SolitaireOne of the mysterious peripatetic, or roving, rocks of Death Valley National Park (see map) in California and Nevada sits at the end of a curved track in a summer 2010 picture.Found in the Racetrack—an aptly named dry lakebed, or playa—the moving rocks have stumped scientists since the 1940s. For instance, the rocks are thought to move as fast as a walking person, but they've never been seen in action. Previous studies have shown that gravity or earthquakes can't explain the objects' movements.Now a student-research project led by NASA's Goddard Space Flight Center has lent support to the idea that, during wintertime, the rocks float down the playa on small "collars" of ice, which form around the stones when lake water flows down the surrounding hills and freezes on the lakebed, according to Cynthia Cheung, a principal investigator for the project. More water flows may allow the ice-collared rocks to "float."A team of undergraduate and graduate students studied data from tiny sensors placed underneath the soil to monitor water flows. The team found that the sensors registered freezing water temperatures in March, which would provide the right conditions for ice collars to form.Even so, the ice theory's not rock solid, Cheung noted: The harsh desert's many microclimates mean that "each rock ... may move by a different force, [and] there may not be one hypothesis that fits all the movements."—Christine Dell'Amore

Photograph courtesy Cynthia Cheung, GSFC/NASA