5. Pottu Codinu, Sardinia
Unlike the other sites on this list, the Pottu Codinu in Sardinia, Italy, isn’t a castle, a temple or a theater. It is far more ancient than the other sites, too. The Pottu Codinu is an ancient burial ground dating to 3,500 BC. The ruins are situated on the back roads of Sardinia, and are completely unmaintained, open to the elements. These ruins aren’t actual buildings, rather, they are man-made caves in the ground.