Machu Picchu is an Incan citadel set high in the Andes Mountains in Peru, above the Urubamba River valley. It was built in the 15th century by the Inca empire and later abandoned. It is renowned for its sophisticated dry-stone walls that fuse huge blocks without the use of mortar. Many intriguing buildings of archaeology are on astronomical alignments and yield stunning panoramic views. Its exact former use remains a mystery. -Nate Loper