Pueblo Bonito, Chaco Canyon
Contrary to many of the small sites previously viewed, this large multi-building site showing numerous kivas [the round structures]. It's a 200+ room Anasazi structure that was built around 900 AD and abandoned around 1200 AD. There are 13 of these "pueblo towns" in the Chaco Canyon region ... all within a few square miles and all deserted at about the same time. The reasons are thought to be an extended 25 year drought coupled with warfare over scarce resources and the emergence of more promising social structures such as the current day Hopi and Zuni pueblos.