Just 30 miles outside Mexico City lies one of the most awe-inspiring ancient cities in the world. At its peak around 500 AD, Teotihuacán was among the largest cities on Earth, home to over 100,000 people. Visitors can climb the massive Pyramid of the Sun and Pyramid of the Moon, and walk the grand Avenue of the Dead — a boulevard that once hummed with the life of a civilization that still mystifies historians.