From the 1940s till recently, the Racetrack Playa, a dry lake bed with a flat surface in Death Valley National Park, was the setting for a “sailing stones” mystery that left people scratching their heads. With years or even decades between each occurrence, an unseen force appeared to move hundreds of rocks across the ground at the same time, leaving long parallel trails in the dried mud. These sailing stones weighed up to 300 kilograms (700 lb) each.
No one had even seen the stones in motion as far as scientists knew. So a team of US researchers decided to investigate in 2011. They set up time-lapse cameras and a weather station to measure wind gusts. Then they installed motion-activated GPS tracking units in 15 limestone rocks and set them on the playa.
It could have been a decade or more before anything happened, but they got lucky. In December 2013, the team was there in person when the stones sailed—and the mystery was solved.
Heavy rain and snow had left 7 centimeters (3 in) of water on the playa. It froze at night into thin sheets of ice that broke up into larger floating panels under the midday sun. Light winds of about 15 kilometers (10 mi) per hour were needed for the accumulated ice to push the rocks across the playa, leaving tracks in the mud beneath the icy surface. The trails became visible months later when the lake bed dried out.
The rocks will only move if conditions are perfect. Not too much wind, sun, water, or ice. Not too little, either. “It’s possible that tourists have actually seen this happening without realizing it,” says researcher Jim Norris. “It is really tough to gauge that a rock is in motion if all the rocks around it are also moving.”
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