The Ancestral Puebloans (also referred to as the Anasazi) built elaborate structures throughout the desert southwest, then mysteriously dispersed and became assimilated into the Zuni, Hopi and Pueblo cultures. Among the ruins left by these cultures are Mesa Verde in southern Colorado and Canyon de Chelly in Arizona.
In my opinion, however, the most spectacular and most interesting site is at Chaco Canyon in northern New Mexico. Unlike many other early structures in the southwest built as cliff dwellings for protection from the elements and possibly hostile tribes, Chaco was constructed as a free-standing, multi-storied city on a wide, flat canyon floor. Chaco was a major trading and religious center and was linked by more than 400 miles of wide and well-planned roads to some 75 communities. Artifacts found at Chaco reveal trade with groups as far south as Mexico.
Chaco thrived from about 900 CE through the late 1100s when the site was abandoned, possibly as a result of a prolonged drought or possibly in keeping with religious guidance to move on to new areas. As Chaco was declining, Mesa Verde arose to the north.
Both Chaco and Mesa Verde exhibit distinctive traits of the Ancestral Puebloan culture. Both have many T-shaped doorways and both contain small and large round kivas, some subterranean, some built above ground. These kivas were meeting places for religious ceremonies and possibly had many other uses as well. Although open to the elements today, kivas were once covered and had small outlets to allow smoke to escape. Shown below is the great kiva at Casa Rinconada, the largest kiva at Chaco Canyon (note the T-shaped entrance on the fall wall).