One of the largest and most famous Chacoan structures is called Pueblo Bonito.[71] It was a town, consisting of a single building, which covered over three acres of ground and contained at least eight hundred rooms. It has been estimated that it could have sheltered 1200 inhabitants, and it was the largest “apartment house” in the world until a larger one was erected in New York in 1882. Building had begun at Pueblo Bonito as early as 919 A.D., but it did not reach its final form until 1067 A.D. or later. It is believed that the more definitely planned settlement may have been the work of new and more progressive people who moved into the area. Pueblo Bonito, as it stands today after archaeologists have cleared away the dust of centuries and exposed it to view, is truly a remarkable structure. Even in ruins, it is not too difficult to picture it as it must have been during those long ago times when it was one of the great cultural centers of the Southwest. On three sides of the center court was the main building, terraced back from a one-story level in front to four stories in the rear. With each succeeding row of rooms the height was increased by one story. Extending from the ends and enclosing the side to the south was a one-story row of rooms. Outside of this single tier was the rubbish heap around which retaining walls were built. The center court contained numerous kivas, and others were incorporated in the building mass. In addition to the regular kivas, whose diameter rarely exceeded twenty-five feet, there have also been found in Chaco Canyon, Aztec, and other sites with Chacoan architecture, big circular structures with diameters of from forty to sixty feet ringed by a concentric row of small rooms. These are known as _Great Kivas_. They are thought to have been religious edifices which served an entire community, while the smaller kivas probably belonged to various clans or societies. Great Kivas, though in a simpler form, were apparently present as far back as Modified-Basketmaker times when most rites were performed in dwellings, but a larger place was needed for ceremonies in which the people of a whole community or district participated.