They are some of the biggest pyramids on the planet - millions of tons of stone and earth towering above the landscape in a massive display of wealth and power. But the pharaohs didn't build these pyramids. This is Mexico - and the truth is, we don't know who built them-or why. Today, they remain as centrepieces of the majestic ancient city of Teotihuacan, the first major metropolis of the Americas. Home to one of the most powerful civilizations of its time, the city rivalled Shakespeare's London in size and outlasted ancient Rome. So why, around 750 AD, did the advanced civilisation that created Teotihuacan suddenly vanish? The identity of its founders, the language they spoke - even the original name of the city - are all unknown. DNA analysis of bodies from Teotihuacan shows they weren't Mayan, Incan or Aztec, but a different civilisation entirely. With none of the macabre murals dedicated to war that are found in other cultures of Mesoamerica, it was assumed to be a peaceful, utopian society. But the latest discoveries are revealing a much darker scenario. In the depths of Teotihuacan's pyramids, experts have uncovered vault after vault filled with curious human remains. Some are filled with women who were buried alive. Others hold the remains of warriors, entombed with their skeletal hands still tied behind their backs, heads missing. And most spectacular of all, in the same vault with ten warriors, two mummies outfitted with all the finery of royalty. With historical recreations and spectacular CGI, Pyramids of Death brings the world of these ancient people to life - from their remarkable feats of construction and engineering to their grisly methods of human sacrifice.