From the World Heritage inscription:
For over 2,000 years, Pueblo peoples occupied a vast region of the south-western United States. Chaco Canyon, a major centre of ancestral Pueblo culture between 850 and 1250, was a focus for ceremonials, trade and political activity for the prehistoric Four Corners area. Chaco is remarkable for its monumental public and ceremonial buildings and its distinctive architecture – it has an ancient urban ceremonial centre that is unlike anything constructed before or since. In addition to the Chaco Culture National Historical Park, the World Heritage property includes the Aztec Ruins National Monument and several smaller Chaco sites managed by the Bureau of Land Management.
Along with Mesa Verde, Chaco Culture is one of the best examples of ancient culture in the United States. Getting to Chaco Culture National Historic Monument is a bit of a challenge. It is a very remote site that required at least 20 miles of travel over unpaved roads. If it has been recently raining, forget about visiting.