Death Valley National Park,
The Mystery: People have long scratched their heads over the "Sailing Stones," which mysteriously move across the sandy playa's surface on their own, leaving visible tracks in their wake.
Fact: Given that these rocks chart a new course once every three years, it's no wonder no one has ever seen them in motion. Some theorize that, in winter, wet clay and strong winds—which can reach speeds of up to 90 mph—are to blame, but no one is 100 percent certain what causes this curious natural (or unnatural?) phenomena.