The stone spheres of Costa Rica - known locally as Las Bolas - have been the focus of University of Kansas researcher John Hoopes, who has evaluated the spheres for protection as a Unesco World Heritage Site.
First discovered by archaeologists in the late 19th Century, some of the spheres date from about AD 600 AD, with many being made after AD 1000 AD but before the 16th Century Spanish conquest.
The stones vary from just a few centimetres in size to more than 2.5 metres in diameter. The largest weighs in at more than 14 tonnes.
More than 300 of the spheres exist; most are made from gabbro, a rock formed from molten magma. The stone, in most cases nearly perfectly spherical, has been worked smooth by pecking, grinding, and hammering with smaller stones.
While many have long been moved from their original homes, some are yet to be fully excavated. Here, a sphere has been found near a ramp signifying an ancient "house mound".
In rare cases, the spheres feature petroglyphs - images carved into the rock, being described here by archaeologist Ifigenia Quintanilla of the Autonomous University of Barcelona.
Despite extensive study into the spheres, their original purpose remains unclear. There are suggestions that some arrangements of the spheres have an astronomical significance.
Because the culture of the people who made the spheres disappeared after the Spanish conquest, the precise purpose and meaning of the balls will remain mysterious - and possibly protected by Unesco.
What are these?