example was the proposed competitive leasing of federal lands in the San Juan Basin for surface coal mining beginning in 1983. As ample coal deposits abut the park, this strip mining threatened the web of ancient Chacoan roads. The year-long "Chaco Roads Project" thus documented the roads, which were later protected from mining. As archaeological significance extends beyond the park's boundaries, the United States Department of the Interior intends to withdraw a 10-mile buffer zone around the park from development. Sites The Chacoans built their complexes along a stretch of canyon floor, with the walls of some structures aligned cardinally and others aligned with the 18.6-year cycle of minimum and maximum moonrise and moonset. Central canyon The central portion of the canyon contains the largest Chacoan complexes. The most studied is Pueblo Bonito. Covering almost and comprising at least 650 rooms, it is the largest great house; in parts of the complex, the structure was four stories high. The builders' use of core-and-veneer architecture and multi-story construction necessitated massive masonry walls up to thick. Pueblo Bonito is divided into two sections by a wall precisely aligned to run north–south, bisecting the central plaza. A great kiva was placed on either side of the wall, creating a symmetrical pattern common to many Chacoan great houses. The scale of the complex, upon completion, rivaled that of the Colosseum. Nearby is Pueblo del Arroyo, which was founded between AD 1050 and 1075 and completed in the early 12th century; it sits at a drainage outlet known as South Gap. Casa Rinconada, isolated from the other central sites, sits to the south side of Chaco Wash, adjacent to a Chacoan road leading to a set of steep stairs that reached the top of Chacra Mesa. Its sole kiva stands alone, with no residential or support structures whatsoever; it did once have a passageway leading from the underground kiva to several above-ground levels. Chetro Ketl, located near Pueblo Bonito, bears the typical 'D'-shape of many other central complexes. Begun between 1020 and 1050, its 450–550 rooms shared one great kiva. Experts estimate that it took 29,135man-hours to erect Chetro Ketl alone; Hewett estimated that it took the wood of 5,000 trees and 50 million stone blocks. Kin Kletso ("Yellow House") was a medium-sized complex located west of Pueblo Bonito. It shows strong evidence of construction and occupation by Pueblo peoples from the northern San Juan