A long-standing Fortean mystery has been the 'wandering stones' of Racetrack Playa in Death Valley, California. It might be time to cross this one off the list though:
Ending a half-century of geological speculation, scientists have finally seen the process that causes rocks to move atop Racetrack Playa, a desert lake bed in the mountains above Death Valley, California. Researchers watched a pond freeze atop the playa, then break apart into sheets of ice that — blown by wind — shoved rocks across the lake bed.
...The researchers began studying the region in 2011, setting up a weather station and time-lapse cameras and dropping off rocks loaded with Global Positioning System (GPS) trackers. The rocks were designed to start recording their position and speed as soon as something made them move.
...When the researchers travelled to the playa in December 2013 to check instruments and change batteries, they found a huge ice-encrusted pond covering about one-third of the 4.5-kilometre-long playa. After several days of camping, they decided to sit above the southern end of the playa on the morning of 20 December. “It was a beautiful sunny day, and there began to be rippled melt pools in front of us,” Richard Norris says. “At 11:37 a.m., very abruptly, there was a pop-pop-crackle all over the place in front of us — and I said to my cousin, ‘This is it.’ ”
They watched as the ice began moving past the rocks, mostly breaking apart but also shoving them gently...when the ice melted away that afternoon, they saw freshly formed trails left behind by more than 60 moving rocks.
The following month, the research team even managed to capture video of the phenomenon occurring:
Whether this is the complete explanation of the wandering stones phenomenon is still unknown - there have been reports of the stones moving during summer months as well, when it's unlikely that ice could form on the playa, leading previous researchers to note that ice "is not a required component or precondition for sliding rock activity".
So while the mystery seems to have largely been solved, there's still a few loose ends that might need cleaning up.