The mythical "sunstone," which legend says the Vikings used to guide them across the high seas, may not be so imaginary after all, according to scientists who believe they've discovered one. Researchers at the University of Rennes in Brittany found the palm-sized calcite crystal in the wreckage of a ship that sank off the Channel Islands and determined it may have actually worked as a navigation aid. "It permits the observer to follow the azimuth of the sun, far below the horizon with an accuracy as great as plus or minus one degree," writes researcher Guy Ropars.