Mesa Verde National Park, Colorado

In the sixth century A.D., the Anasazi, or "Ancient Ones," established villages on the high, flat land in southwestern Colorado. At first, they lived in caves and in low shelters built over pits on the mesa top. In the late 1100s they began constructing multi-story stone apartment houses or pueblos tucked in on ledges and under rock overhangs. One particular complex, today called the Cliff Palace, contained more than 200 rooms: religious spaces called kivas, rooms for sleeping and crop storage. The reason the Anasazi moved to these cliffs is not known; perhaps it was for shelter from their enemies. Also unknown is why they abandoned their new homes scarcely 100 years later at the end of the 13th century -- perhaps due to war, drought or their own depletion of resources. (Inscribed in 1978)