In the cloud forests of Peru the stone walls of a mysterious mountain top fortress rise out of the jungle. These 60 foot walls are filled with the bones of the Chachapoya, the Cloud Warriors, who lived high in the Andes from A.D. 800 to the mid 1500s. Only after an intense struggle did the powerful Incan empire gain control of the fiercely independent Chachapoya tribes. But did the Inca ever conquer the Chachapoya stronghold of Kuelap? Archaeologists at Kuelap have uncovered hundreds of elaborate burial sites throughout the settlement that reveal tantalising clues about the identity of the Chachapoya people and how and why they built such a massive fortress. But the biggest mystery of all is why did the entire Chachapoya civilisation abruptly disappear in the 16th century? A team of archaeologists led by Alfredo Narvaez examine some startling evidence over 80 bodies piled at one end of Kuelap, stacked between houses, lying where they fell, and not carefully buried like all of Kuelap’s other bones. These people were killed when no one was left to bury them. Alfredo and his team have uncovered the largest mass grave in Peru and believe they may have finally found the secret to Kuelap’s last stand.