Tucson is a city in and the county seat of Pima County, Arizona, United States. The city is  located 118 miles (188 km) southeast of Phoenix and 60 miles (98 km) north of the U.S.-Mexico border. A major city in the Arizona Sun Corridor, Tucson is the largest city in southern Arizona, the second largest in the state after Phoenix. It is also the largest city in the area of the Gadsden Purchase. Tucson is home to the University of Arizona. Roughly 150 Tucson companies are involved in the design and manufacture of optics and optoelectronics systems, earning Tucson the nickname Optics Valley.