They are some of the biggest pyramids on the planet, millions of tons of stone and earth towering above the landscape in a display of massive wealth and power. But it wasn't the pharaohs that built these pyramids. This is the majestic ancient city of Teotihuacán, Mexico, home to one of the most powerful civilizations of its time. But why, around 750 AD, did the advanced civilization that created Teotihuacán suddenly vanish? The identities of its founders, the language they spoke and even the original name of the city are all unknown. DNA analysis of bodies from Teotihuacán shows they weren't Mayan, Incan or Aztec, but an entirely different civilization. It was assumed to have been a peaceful, utopian society, but the latest discoveries are revealing a much darker scenario. In the depths of Teotihuacán's pyramids, experts have uncovered vault after vault filled with curious human remains.