The stone spheres of the Diquís

Fifteen-ton stone balls, and no one knows why

More than 300 stone spheres lie across the Diquís Delta of southern Costa Rica and on Isla del Caño, the best-known stone sculptures of the Isthmo-Colombian region, known locally as bolas de piedra. They range from a few centimetres to over 2 metres across and weigh up to 15 tons. Most are of gabbro, the coarse-grained equivalent of basalt, quarried in the hills several kilometres from where the finished spheres lie, and a dozen or so each are of shell-rich limestone and of sandstone. They seem to have been made by hammering natural boulders with other rocks and polishing them with sand, and the finish varies from rough to remarkably true.

They are attributed to the Diquís culture. They are thought to have been made from around 600 CE, most after 1000, all before the Spanish conquest, though with so few left where they were placed, dates are hard to pin down. They were set in lines along the approaches to chiefs' houses, and what they meant remains uncertain; one hypothesis sees in them the sun and moon at their various stages.

The modern story began in the 1930s, when the United Fruit Company cleared the jungle around Palmar Sur for banana plantations. Bulldozers shoved the spheres aside, and workmen who had heard stories of gold hidden inside drilled holes in some and blew them apart with dynamite before the authorities stepped in. Some of the broken ones have been pieced together at the National Museum of Costa Rica in San José, and many others ended up as garden ornaments. Doris Stone, daughter of a United Fruit executive, made the first scientific study and published it in 1943, and Samuel Lothrop of Harvard's Peabody Museum mapped the spheres where they still stood.

In 2014 UNESCO listed four settlements, Finca 6, Batambal, El Silencio and Grijalba-2, as the Precolumbian Chiefdom Settlements with Stone Spheres of the Diquís, dating them to 500 to 1500 CE and noting spheres from 0.7 to 2.57 metres. Thick sediment kept them buried for centuries and spared them the looting that befell most of Costa Rica's sites. Finca 6, on old United Fruit land near Palmar Sur, is the only one with spheres still in their lines, and El Silencio holds the largest single sphere ever found. In July 2014 Costa Rica also declared the spheres a national symbol.