Using fire-starting rock such as flint, Stone Age people originally created the stones to serve as axes. But the Vikings, whose Iron Age heyday lasted from about A.D. 800 to 1050, saw the primitive tools as lightning repellent.

Because the axes predate the Viking age by thousands of years, archaeologists have long seen the stones as random artifacts, perhaps stirred up from earlier, lower burials or dropped in centuries after the Viking era.

But now "we have made enough discoveries of Stone Age artifacts in younger graves to say that they make a clear pattern," archaeologist Eva Thäte, of the University of Chester in the U.K., said in a statement.

(Also see "Viking Weapon-Recycling Site Found in England?")

To solve the thunderstone