The site of a great Ancestral Pueblo center in the 11th and 12th centuries AD, the ruins in Chaco Canyon look like a city to some archaeologists, a ceremonial center to others. Chaco and the people who created its monumental great houses, extensive roads, and network of outlying settlements remain an enigma in American archaeology. Two decades after the latest and largest program of field research at Chaco (the National Park Service's Chaco Project from 1971 to 1982) the original researchers and other leading Chaco scholars convened to evaluate what they now know about Chaco in light of...