For years scientists have theorized about how large rocks — some weighing hundreds of pounds — zigzag across Racetrack Playa in Death Valley National Park, leaving long trails etched in the earth. Now two researchers at the Scripps Institution of Oceanography at the University of California, San Diego, have photographed these "sailing rocks" being blown by light winds across the former lake bed. Richard Norris and James Norris said the movement is made possible when ice sheets that form after rare overnight rains melt in the rising sun, making the hard ground muddy and slick. On Dec. 20, 2013, the...