Popping up in yards across Costa Rica, the huge, stone Diquís Spheres might be seen as simply a landscaping trend, but in fact the rounded stones are mysterious artifacts that were created centuries ago in great numbers, but for unknown reasons.

In the 1930s, workers from the United Fruit Company, clearing land in the Diquís Valley of Costa Rica, began unearthing large numbers of almost perfectly round stone spheres. The largest of these apparently man-made balls is over two meters in diameter and weighs over 16 tons. No one is sure exactly when or how they were made, or for what reason, but, according to University of Kansas archeologist John Hoopes, “The balls were most likely made by reducing round boulders to a spherical shape through a combination of controlled fracture, pecking, and grinding.” They were likely the product of an extinct civilization of people that existed in the area between 700 CE and 1530 CE.

Today, virtually all of the spheres have been taken from their original locations. Many are prized lawn ornaments across Costa Rica. A collection of six now resides in the courtyard of Costa Rica’s National Museum in San Jose.