The city in the sky – Machu Picchu – is one of the most beautiful World Heritage Sites and the main symbol of the Inca Empire. The continuing excavations at Machu Picchu have brought to light many of the secrets of Incan prosperity. The city itself is built on a steep Andean mountain ridge, and scholars have often wondered how and why the Incans built it in this harsh environment. This episode explores the Incan secrets in building this magnificent city.