It is perched on a mountain ridge in the Andes, 1,600 feet above a river valley - and 8,000 feet above sea level. A city of stone built without mortar, the wheel, or iron tools, it is so perfectly constructed that the thinnest knife blade cannot fit between the joints in the massive granite blocks that make up most of the structures. But why was it built? And why was it abandoned, apparently only a couple hundred years after it was built? Head high into the Andes to explore the mysterious cloud city of the Inca, and meet the archeologists and scholars who are leading the efforts to reveal its many secrets. Well above the surrounding cloud forest, Machu Picchu likely had no commercial, military or administrative use, but many theorists now believe that it was a royal estate and religious retreat, and we'll examine the evidence that points to this conclusion.