I want a second the recommendation to visit Teotihuacán, but also add that really not anywhere close to half of it has been excavated yet. The impressive ceremonial core of the city, with its pyramids and dramatic Avenue of the Dead, along with a couple of nearby apartment compounds, only represents perhaps 10% of the entire city. the main reason the rest has not been excavated is that it is underneath the modern-era towns of San Juan and San Martin lying to either side of the ceremonial center.