From UNESCO:

Precolumbian Chiefdom Settlements with Stone Spheres of the Diquis includes four archaeological sites located in the Diquis Delta in southern Costa Rica, which are considered unique examples of the complex social, economic and political systems of the period between 500-1500 AD. They contain artificial mounds, paved areas, burial sites and, most significantly, a collection of stone spheres, between 0.7m and 2.57m in diameter, whose meaning, use and production remain largely a mystery. The spheres are distinctive for their perfection, their number, size and density, and their placement in their original locations. Their preservation from the looting that befell the vast majority of archeological sites in Costa Rica has been attributed to the thick layers of sediment that kept them buried for centuries.

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