Machu Picchu has stood empty above the Urubamba valley for centuries, yet its stonework still holds together without mortar after repeated earthquakes and landslides. This DW film follows researchers examining how the Incas, working without a conventional writing system and recording information instead through knotted quipu cords, planned and built a complex of several dozen structures spread across ten hectares at over 2,400 meters altitude. Engineers and archaeologists explain the drainage systems that keep the site from washing away in heavy rain, and astronomers walk through evidence that certain buildings were aligned to track the sky. Spectacular 3D animations reconstruct construction techniques that left no written record, showing how interlocking stones were cut and fitted to survive on unstable mountain terrain. Interviews with specialists studying the site's engineering and its place in Inca cosmology fill out the picture of a 15th-century city built to last, discovered by the outside world only at the start of the 20th century and now visited by roughly 1.5 million people a year.