Scattered about the Racetrack Playa in Death Valley California are large rocks with mysterious zig-zagging trails behind them. The trails are evidence that these rocks have moved, as much as 820 feet in a single winter, leaving zig-zag trails across the ground. There are over 150 such animate rocks. And yet, no one has ever seen them move.
In 1972 a team of scientists set out to unravel the mystery. They named a group of stones and did surveys of the area over a seven-year period. A 700-pound block dubbed Karen, which didn't move at all while under study, was entirely missing when they returned years later. A sighting of the 700-pound Karen was later made over half a mile from the survey site.
Teams have continued to study the phenomenon and believe that winter ice and strong winds, which can reach upwards of 90 miles per hour, are responsible for the stones' movement.
More photos of The Sailing Stones of Racetrack Playa can be seen on Atlas Obscura.
For the rest of the story: http://www.slate.com/blogs/atlas_obscura/2013/05/29/the_sailing_stones_of_racetrack_playa_leave_trails_of_movement_yet_no_one.html