This is the most extensive and well-developed example of pre-historic cliff dwellings in the United States. The ruins trace the development of the Anasazi as they moved from the early pithouses on the mesa-tops to the large apartment complexes built in caves on the cliff walls of the canyons. More than 4,000 excavated and unexcavated mesa-top and cliff-side sites dot the more than 50,000 nominated acres of the park. Declared a National Park in 1906, Mesa Verde is also important as a landmark of cultural preservation in the United States. It was one of the nation's earliest attempts to preserve a large tract of archaeological ruins and a collection of artifacts through federal legislation.