In 1911 Yale University Professor Hiram Bingham, searching for the lost Inca capital of Vilcabamba, paid a Peruvian guide to lead him to a nearby ruin. The guide took him 2,000 feet (610 meters) up a precipitous slope—and straight into the “lost” city of Machu Picchu (shown here in the first of a series of photographs from the period).

Arguably the greatest archaeological site in the Americas, Machu Picchu remains a mystery. Some scholars believe it to be the birthplace of the Inca Empire. Others see a ceremonial center or military citadel.