Today, the ancient mountain city of Machu Picchu is shrouded in mystery. Built by the Incas sometime in the middle of the 15th century, it was only occupied for about 100 years – and then the people vanished. For centuries, we knew nothing about the existence of this place. The city lay undiscovered – hidden from the world – for almost 400 years until an American historian stumbled across it in 1911. What, exactly, is the meaning of this place? Why did the Incas originally build it?