Mystery of Death Valley's Sliding Rocks Solved. Death Vally. “We expected to wait five or ten years without anything moving,” Norris says in a news release, “but only two years into the project, we just happened to be there at the right time to see it happen in person.” On December 2013 trip, the playa was covered with a pond of water about seven centimeters deep. “On Dec. 21, 2013, ice breakup happened just around noon, with popping and cracking sounds coming from all over the frozen pond surface,” Norris recalls. Shortly afterwards, the rocks started moving. Moving the rocks requires a rare sequence of events, they discovered. First, the playa fills with water deep enough to form floating ice during the winter -- but shallow enough to expose the rocks. When temperatures plummet at night, thin sheets of “windowpane” ice form: At three to six millimeters, it's just thin enough to move freely but thick enough to maintain strength. Then on sunny mornings, the ice covering the pool begins to melt, breaking up into large floating panels that move across the playa with light winds of up to five meters per second. The floating ice push the rocks in front of them at low (almost imperceptible) speeds of up to five meters per minute, along trajectories determined by wind and the water flowing underneath -- leaving trails in the soft mud below. Death Vally.