Looking north from the south shore of Racetrack Playa on December 20, 2013 at 3:15 pm. Steady, light wind has blown water to the northeast exposing newly formed rock trails / 2014 Norris et al., PLoS ONE

Hundreds of mysterious engraved trails have been stumbled upon at the Racetrack Playa of Death Valley in California. There’s almost always a rock sitting at the end of the track, but no one has actually seen a rock -- not a pebble nor a boulder -- sliding on this nearly flat, dry mud surface. What gives? Several decades of speculation range from powerful winds and ice flotation to dust devils and slick films.

Now, for the first time ever, researchers have observed the sliding rocks in action. Finally! Under just the right conditions, thin sheets of ice blown by light winds push the rocks across the dry lake. The work, which turned out not to be “the most boring experiment ever,” was published in PLoS ONE this week.

During the winter of 2011, a team led by Richard Norris from Scripps Institution of Oceanography brought in 15 rocks fitted with motion-activated GPS units, and they monitored them with the help of a high-resolution weather station and time-lapse cameras overlooking the southeast corner of the playa.

“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.

Here’s one of the GPS rocks with its trail on a cold morning when the pond surface is still covered with ice.

Rocks stayed in motion anywhere between a few seconds to 16 minutes. In the two and a half months that the pond existed, some rocks moved up 224 meters, with the largest rock movement involving more than 60 rocks at a time. In one event, rocks 300 meters apart moved simultaneously, traveling 60 meters before stopping.

Images: 2014 Norris et al., PLoS ONE (top), Richard Norris (middle), Mike Hartmann (below)