It is widely speculated that the regal mountaintop city of Machu Picchu, perched high in Peru's Andes Mountains, served as a royal estate and spiritual haven during the height of the Incan Empire. But the true purpose of the walled city remains shrouded in mystery, and it is alternately held to have been a fortress, an observatory, and a palace. The city was known in legend only for centuries after falling diseases brought by the Spanish conquistadors, as well as internal strife, caused the Empire's downfall. Rediscovered in 1911 by Yale professor Hiram Bingham, who was searching for the still-mythical Vilcabamba, Machu Picchu has since revealed many of its secrets to the teams of researchers who have culled its every corner for evidence of the past. Footage of the earliest expeditions to the site, as well as interviews with experts and explorations of the city today, allow a look inside this most mysterious of relics.