Chaco Canyon is arguably the most remarkable archaeological site in the United States. The biggest building in the area is now known as Pueblo Bonito. It has a semicircular shape, with a diameter of about 500 feet. It covers more than 4 acres, and in some places was five stories high. It contained over 500 rooms and 40 kivas (circular rooms for religious uses). Construction was begun about AD 800, and continued until about AD 1250. The walls were of stone, laid in small pieces and covered covered by plaster. The roofs were supported by timbers. The nearest large trees were more than 60 miles away. The Chacoans brought thousands of large timbers, and without the use of the wheel or beasts of burden.
This was the largest man-made structure in what is now the US, for 400 years after it was built. This is not the only major structure in the area. These are now called "great houses" and there are several in Chaco Canyon itself. However, there were once 150 more great houses scattered through northwest New Mexico, with some in Arizona and Colorado. There were more than 400 miles of good roads connecting the great houses.
Even though they had no written language, The Chacoans had an excellent grasp of astronomy and surveying. The great houses are aligned (quite accurately) with the cardinal points of the compass, and the Chacoans also determined the equinoxes with great accuracy.
Yet, fascinating as this site is, it is but little visited. It is out of the way, in northwestern New Mexico, at 6,200 feet above sea level, and requires traversing at least 18 miles of washboard dirt roads. The site is managed by the National Park Service as Chaco Culture National Historical Park.