The sailing stones (sliding rocks, moving rocks) are a geological phenomenon where rocks move in long tracks along a smooth valley floor without human or animal intervention. They have been recorded and studied in a number of places around Racetrack Playa, Death Valley, where the number and length of travel grooves are notable. The force behind their movement is not understood and is subject to research.

More Info:
http://en.wikipedia.org/wiki/Sailing_stones

The true mystery of the world is the visible, not the invisible.
— Oscar Wilde