The Inca were brilliant builders. They cut huge stones so perfectly that you cannot slide a piece of paper between them - and they did it without metal tools or wheels. Some of the stones at Machu Picchu weigh more than a small car. Many of the walls are still standing exactly where the Inca placed them.
Machu Picchu has houses, temples, fountains, and long flat 'steps' on the hillside called terraces, where farmers grew potatoes and maize. There is even a stone carved like a sundial, called Intihuatana - 'the hitching post of the sun' - which the Inca used to keep track of the seasons.
The site was hidden by thick cloud forest for centuries. In 1911, an American teacher called Hiram Bingham was led there by a local farmer, and the rest of the world heard about Machu Picchu for the first time. Today it is one of the most famous places on Earth - a UNESCO World Heritage Site.
Getting there is still an adventure. The most popular way is the 'Inca Trail', a four-day walk over high mountain passes. The last bit goes through a stone gateway called Inti Punku - the Sun Gate - so the city suddenly appears below you as the morning mist lifts.