Mesa Verde

More than 5,000 recorded sites in one park, including about 600 cliff dwellings built into alcoves under the canyon rims, of which Cliff Palace is among the largest in North America. Ancestral Puebloans put up the first pueblos on the mesa top after about 650 and only moved into the cliffs at the end of the twelfth century, which is late and is part of the puzzle. Then they left. Tree-ring dating puts the last beam cut on the mesa at 1281, and by 1285, after a run of severe and prolonged droughts and the social strain that came with them, they had migrated south into what is now Arizona and New Mexico. Their descendants are the modern Pueblo nations, so this is a place people moved away from, not a place that vanished. UNESCO World Heritage since 1978. The cliff dwellings are ranger-guided and ticketed, and tickets go fast in summer. Montezuma County, Colorado.