Scattered throughout the island of Gotland, in the middle of the Baltic Sea, are thousands of stones with strange grooves or furrows cut into its smooth, hard surface. The grooves always occur in groups, cut side by side and are of varying length, width and depth.

At first glance, it appears as if someone had been sharpening their axes or swords on them. That was the general opinion when the grooves were widely reported in the mid-19th century. Consequently, the grooves were called "sharpening stones". But soon scholars began to have doubts about their origin, since the shape and size of the grooves made them unfit for sharpening swords. Someone pointed out that stone-age swords, and even those from the Middle Ages and Viking Age, were too wide to fit into the grinding grooves.

Photo credit: Sören Gannholm