MARCHING STONES
For almost a century one of America's strangest mysteries has been found in remote Death Valley, California. It's there -- actually at a specific dry lakebed called Racetrack Playa -- that stones are claimed to mysteriously move on their own, when no one is looking. The phenomenon occurs in a handful of other places as well, though none are as well known as those in Death Valley. They moved very slowly, in some cases only a few inches over months or years, but their trails can clearly be seen in the dried mud behind them.
Over the years many explanations have been offered, ranging from hoaxing to aliens to some sort of localized, unknown magnetic effect.
Others have suggested that the area's strong winds might move the stones, but that doesn't explain why they'd move at different rates and sometimes in different directions. For many years the best scientific explanation was that the rocks moved due to a specific combination of wind, temperature, and water. Racetrack Playa is in a desert, but sometimes collects water from rain and melting snow, providing a slick surface over which the stones might move.
The mystery was finally solved in 2014 when a team of researchers set up cameras over the area and measured the rocks' movement patterns. They concluded that the stones moved under just the right conditions when ice formed under the rocks and moved them, usually only a little bit at a time: "moving sheets of ice tens of meters in extent but only a few millimeters thick are clearly effective at moving rocks in their path." The study, "Sliding Rocks on Racetrack Playa, Death Valley National Park: First Observation of Rocks in Motion" was published on Aug. 27, 2014 in the open-access journal PLoS ONE.
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