While clearing the jungle for banana plantations in 1940 in Costa Rica's Diquis Delta region, employees of the United Fruit Company uncovered numerous large stone spheres partly buried in the forest floor.

Almost immediately, the mysterious spheres became prized ornaments, ending up on the front yards of government buildings and fruit company executives throughout Costa Rica. Many spheres were also broken or damaged and others were dynamited in a time when few realised their archaeological value.

According to John Hoopes, associate professor of anthropology and director of the Global Indigenous Nations Studies Program, around 300 spheres are known to exist, with the largest weighing 16 tonnes and measuring eight feet in diameter, and the smallest being no bigger than a basketball. Almost all of them are made of granodiorite, a hard, igneous stone.

Since their discovery the true purpose of the spheres, which still eludes experts, has been the subject of speculation ranging from theories about the balls being navigational aids, to relics related to Stonehenge, the product of an unknown ancient civilization or visits from extraterrestrials.

Part of the mystery surrounds the way in which they were created as the near-perfect spheres appear to have come from a quarry that was more than 50 miles away and they were created in a time in which metal tools had apparently not been invented yet as it is estimated that the stones were made around 600 AD. However, the dating method for stones is speculative in itself as it really only reveals the latest use of the spheres not when they were first created.

"These objects can be used for centuries and are still sitting where they are after a thousand years. So it's very difficult to say exactly when they were made," explained Hoopes.

However, the biggest mystery remains what they were used for. "We really don't know why they were made," Hoopes said. "The people who made them didn't leave any written records. We're left to archaeological data to try to reconstruct the context. The culture of the people who made them became extinct shortly after the Spanish conquest. So, there are no myths or legends or other stories that are told by the indigenous people of Costa Rica about why they made these spheres."

Much like the Easter Island moai, one theory assumes that the spheres were simply status symbols. The stones, which are now protected by UNESCO, also might have been arranged into massive patterns that had astronomical significance as many of the balls were found to be in alignments, consisting of straight and curved lines, as well as triangles and parallelograms. Since almost every sphere has been moved from its original location, however, researchers are sceptical that the true meaning of the spheres will ever be discovered.