The ancient city of Teotihuacan, in the Valley of Mexico, is well known as a large urban center. Established around 100 BC, Teotihuacan grew to be one of the largest cities on the planet by 500 AD, with more than 100,000 people living in the center and the surrounding neighborhoods. Religious buildings like the Pyramid of the Sun and the Pyramid of the Moon are still visited today, and the city boasted multi-level apartment buildings to house its massive population. Many of these people came to Teotihuacan from other parts of the Mesoamerican world, and a new DNA study of the neighborhood of Teopancazco is revealing the genetic diversity of the immigrants.