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, which is 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)
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