Machu Picchu

The Inca citadel perched high in the Peruvian Andes/Ancient Structures

Built around 1450 CE and abandoned roughly a century later during the Spanish conquest, Machu Picchu was unknown to the outside world until 1911. Sitting at 2,430 meters above sea level, its dry-stone walls fit together so precisely that you can't slip a knife blade between them. The site includes about 200 structures and an agricultural terrace system that would make modern landscapers weep with envy.