In the 1930's workers from the United Fruit Company, who were clearing land in the Diquís Valley Delta region of Costa Rica, began finding large numbers of almost perfectly round stone spheres.

The spheres, which range from a few kilograms up to around 16 tonnes, are accurately made, presumably by chipping.

About 300 have so far been found so far. They have been carbon-dated as between 500 A.D. > 1,500 A.D.

Most are made from diorite, a rock similar to granite.

The reason for their manufacture, their function, and the history of the people(s) who made them is unknown.

More info : Unesco (.pdf, 600 pages)

Also see : Carved stone balls (Scotland)plugin-autotooltip__plain plugin-autotooltip_bigCarved stone balls (Scotland)

More than 400 examples of Scottish Carved Stone Balls have been found. They are believed to have originated in the late Neolithic and the Bronze Age - i.e. thought to be more than 5,000 years old.

Most have been found in north-east Scotland, mainly Aberdeenshire. They usually range in size from 70 to 90mm diameter, and are hand-carved with varying numbers of symmetrically-arranged protruding bumps on the surface (from 3 to over 150) .