Southern Arizona is a region of the United States comprising the southernmost portion of the State of Arizona. It sometimes goes by the name Gadsden or Baja Arizona, which means "Lower Arizona" in Spanish. Geography Although Southern Arizona's boundaries are not well-defined, it is generally considered to include all areas south of the Gila River but sometimes only Cochise County, Pima County and Santa Cruz County, anchored by the city of Tucson. Other cities and large towns in Southern Arizona include Ajo, Casa Grande, Gila Bend, Oro Valley, Sierra Vista, Yuma, and the border cities of Nogales and Douglas. Furthermore, the populated areas of Southern Arizona include the major U.S. Army post of Fort Huachuca and Davis–Monthan Air Force Base of the U.S. Air Force. The most major scientific site of Southern Arizona is the set of several astronomical observatories of the Kitt Peak National Observatory, a reasonable distance west-southwest of Tucson. Southern Arizona is the location of several large national monuments protecting the scenery, wildlife, and archaeological sites of Southern Arizona, and the Saguaro National Park, which stands on two large sections of land, one west of the Tucson metropolitan area and the other one east of Tucson. The region includes several small mountain ranges including the Chiricahua Mountains, Huachuca Mountains, Santa Ritas, the Santa Catalinas, the Rincons, the Piñalenos, and others. As surprising as it might seem, some of these mountains are high enough, cold enough, and wet enough in the wintertime to provide alpine skiing at regular ski resorts, with ski lifts, not very distant from cities such as Tucson. Transportation Significant distances requiring transportation in Southern Arizona are generally traveled by highway and the railroad. Southern Arizona is the location of the major transcontinental Interstate highway Interstate 10 from the border with New Mexico westward through Tucson, and then continuing northwest via Casa Grande to the Phoenix–Scottsdale–Mesa metropolis. Also, running westward from Casa Grande is Interstate 8, which goes via Yuma into California, crossing the Colorado River. Finally, the short Interstate 19 runs south from Tucson to Nogales, Arizona, and a major border crossing into Mexico. In addition, the much older Federal highway, U.S. Route 80, crosses Southern Arizona from east to west from the New Mexico border, nearly to the California border, and formerly into California to the Pacific Ocean. However, its westernmost stretch has been replaced by Interstate 8, which was built on top