The mystery of the roaming rocks of Death Valley’s Racetrack Playa may be at an end. Since their discovery in the 1940s, researchers have speculated about what conditions on the playa could cause 15+ kg rocks to slide tens or hundreds of meters across the dry lakebed. But the rare nature of the movement and the remoteness of the location had prevented direct observation of the phenomenon until last December when a research team caught the rocks in motion (see the timelapse animation above or the source video). Winter rain and snow had created a shallow ice-encrusted pond across the playa by the time the researchers arrived to check their previously installed equipment. Late one sunny morning, the melting ice, only millimeters thick, cracked into plates tens of meters wide and began to move under the light breeze (~4-5 m/s). Despite its windowpane-like thickness, the ice pushed GPS-instrumented rocks up to hundreds of meters at speeds of 2-5 m/min. It took just the right mix of conditions–sun, wind, snow, and water–but the two ice-shoving instances the team observed go a long way toward explaining the sailing rocks. (Image credits: R. Norris et al.; J. Norris, source video; NASA Goddard; via Discover and SciAm)