Matthew Wills at JSTOR Daily:
Cliff dwellings of the Ancestral Puebloans—the Anasazi to the Navajo Diné—haunt the desert Southwest. Mesa Verde, Chaco, Canyon de Chelly, and many other sites and artifacts in the Four Corners region stand testament to six centuries of residency. However, they tell us little of the people that came and then went, leaving buildings, pottery, and bones behind. There have been more than 4,000 habitation sites detected in the Mesa Verde region of southwestern Colorado. Population estimates for this area alone in the mid-1200s CE run up to 19,200 people.
Yet these people abandoned the houses and villages more than two hundred years before the first Europeans arrived in the region. Did those newcomers, as many have since, gaze upon the sandstone, timber, and adobe constructions and wonder: who built these, where did they go, why did they go?
more here.