The Inca city of Machu Picchu in Peru has been uninhabited for centuries. But it is nevertheless well preserved. Discovered at the beginning of the 20th century, it attracts around 1.5 million visitors from all over the world every year. At an altitude of more than 2,400 meters, the Incas succeeded in building a masterpiece of architecture and engineering. Located on a mountain ridge in the Andes, it is a place in perfect harmony with nature. The several dozen buildings of Machu Picchu cover 10 hectares. Their interlocking stone construction has withstood centuries of heavy rainfall and frequent landslides. The Incas built the city in the 15th century. They did not have a writing system in the conventional modern sense. Instead, they developed a unique recording system consisting of knotted strings, known as "quipus."