I’m gonna quote Wikipedia, because I couldn’t do better myself, so here it goes: The sailing stones (sliding rocks, moving rocks) are a geological phenomenon where rocks move in long tracks along a smooth valley floor without human or animal intervention. They have been recorded and studied in a number of places around Racetrack Playa, Death Valley, where the number and length of travel grooves are notable. The force behind their movement is not understood and is subject to research.
Racetrack stones only move every two or three years and most tracks develop over three or four years. Stones with rough bottoms leave straight striated tracks while those with smooth bottoms wander. Stones sometimes turn over, exposing another edge to the ground and leaving a different track in the stone's wake.
Sliding rock trails fluctuate in direction and length. Some rocks which start next to each other start out traveling parallel, but one may abruptly change direction to the left, right, or even back the direction it came from. Length also varies because two similarly sized and shaped rocks could travel uniformly, then one could burst ahead or stop dead in its track.
Geologists Jim McAllister and Allen Agnew mapped the bedrock of the area in 1948 and made note of the tracks. Naturalists from the National Park Service later wrote more detailed descriptions and Life magazine featured a set of photographs from The Racetrack. Speculation about how the stones may move started at this time. Various explanations have been put forward over the years that have ranged from the supernatural to the very complex.
Most hypotheses favored by interested geologists posit that strong winds when the mud is wet are at least in part responsible. Some stones weigh as much as a human, which some researchers, such as geologist George M. Stanley, feel is too heavy for the area's wind to move. They maintain that ice sheets around the stones either help to catch the wind or move in ice floes.
Researches studied some stones for several years. Ten of the initial twenty-five stones moved in the first winter with Mary Ann (stone A) covering the longest distance at 212 feet (64.5 m). Two of the next six monitored winters also saw multiple stones move. No stones were confirmed to have moved in the summer and some winters none or only a few stones moved. In the end all but two of the thirty monitored stones moved during the seven year study. At 2.5 inches (6.5 cm) in diameter Nancy (stone H) was the smallest monitored stone. It also moved the longest cumulative distance, 860 feet (262 m), and the greatest single winter movement, 659 feet (201 m). The largest stone to move was 80 pounds (36 kg).
Karen (stone J) is a 29 by 19 by 20 inch (74 by 48 by 51 cm) block of dolomite and weighs an estimated 700 pounds (about 320 kg). Perhaps not surprisingly Karen didn't move during the monitoring period. The stone may have created its 570 straight and old track from momentum gained from its initial fall onto the wet playa.