The Racetrack Playa is a flat, dry lakebed in the middle of a desert with the lowest elevation in North America. It's also home to mysterious "sailing stones" that have baffled visitors since the early 1900s. To an untrained observer, it looks like these stones have moved across the surface of the lakebed entirely on their own, leaving tracks up to 1,500 feet long without any sign of human or animal interference. Many theories have been offered to explain this phenomenon, including strong winds, the pull of the Earth's magnetic field, a clever prankster - or, once again, aliens.

But thanks to a devoted team of scientists, we now know why these stones move. In 2011, researchers from the Scripps Institute of Oceanography attached GPS devices to 15 rocks and left to monitor them. Two years later, they returned to the site and were lucky enough to witness the phenomenon in person. When Death Valley receives a rare winter rainstorm, water can pool on the flat lakebed and freeze overnight, creating large panes of ice around the rocks. In the morning, the ice thaws and cracks into large sheets, and a light gust of wind is all that's needed to move the ice across the lakebed's surface. The ice sheets push the rocks across the lakebed and then melt, leaving nothing behind but the rock's tracks. The scientists called the phenomenon "ice shoving." One of them jokingly described the study as "the most boring experiment ever."