Picture bulldozers clearing a Costa Rican jungle for a banana plantation in the 1930s when metal screeches against rock. They have hit a massive, perfectly smooth, man-made stone sphere, some weighing up to 15 tons, with no clue who made them or why.

This episode investigates the Diquis stone spheres, over 300 gabbro petrospheres carved by a vanished culture that left no written records. We separate the genuine engineering marvel from the alien and Atlantis myths, and confront the tragic loss of context that destroyed our best chance to understand them.

• How a stone-age culture shaped impossibly hard gabbro using only harder rocks, sand, and water, then hauled it kilometers through swamp
• Why archaeologists must rely on stratigraphy to date the spheres, since you cannot carbon-date a rock
• The destruction: bulldozing ...