Tiwanaku
This unsuspecting hole was at the edge of a massive 'temple' area. The guide demonstrated that it was modelled to the human ear. If he talked into one side, it was amplified on the other. If we went around that same side and put our ear to it, while he was far away where we couldn't hear him.. we could actually hear him through the 'ear hole', it sounded like listening to him through ear phones.