Death Valley's Mysterious Sailing Stones

Ordinarily, I don't like to borrow posts from other sites. But I didn't think I could improve upon the caption written by the photographer who took the picture shown above . . .

Deep in the heart of the California desert lies one of the natural world's most puzzling mysteries: the moving rocks of Death Valley. These are not ordinary moving rocks that tumble down mountainsides in avalanches, are carried along riverbeds by flowing water, or are tossed aside by animals. These rocks, some as heavy as 700 pounds, are inexplicably transported across a virtually flat desert plain, leaving erratic trails in the hard mud behind them, some hundreds of yards long. They move by some mysterious force, and in the nine decades since we have known about them, no one has ever seen them move.

Think about that last sentence for a while. More information and pictures at Wikipedia.