Knowledge kept from us: walls of mega stones slotting perfectly together in Peru. 

Saqsaywaman is a citadel on the northern outskirts of the city of Cusco, Peru, the historic capital of the Inca Empire. It is built of dry stone walls constructed of huge stones. The workers carefully cut the boulders to fit them together tightly without mortar. The site is at an altitude of 3,701 m (12,142 ft).