group built soil conservation devices, planted trees, and improved roads and trails. 1941 Heavy rains cause Threatening Rock to fall, destroying ~60 rooms at Pueblo Bonito. 1960 Floors excavated at Una Vida 1971-1982 The Chaco Project, conducted by the National Park Service and the University of New Mexico, surveys and excavates Chaco Canyon 1976-1978 Fourteen rooms at Pueblo Alto excavated by the Chaco Project 1980 Chaco Canyon National Monument is renamed Chaco Culture National Historical Park with 13,000 acres (53 km2) added. The Chaco Culture Archaeological Protection Site program is created to protect Chacoan sites. 1982 NASA's Thermal Infrared Multispectral Scanner (TIMS) detects over 200 miles of a prehistoric (AD 900 or 1000) road system in Chaco Canyon, as well as walls, buildings, and agricultural fields. 1983 Dean and Warren estimate 200,000 trees were used to build great houses. 1987 Chaco Culture National Historical Park is designated a UNESCO World Heritage Site. 21st century 2001 Two-thirds of large roof timbers traced to Chuska Mountains and one-third to San Mateo Mountains. References Chacoan Chacoan Chaco Canyon