Pittsburgh ( PITS-burg) is a city in Allegheny County, Pennsylvania, United States, and its county seat. The city is located in southwestern Pennsylvania at the confluence of the Allegheny River and Monongahela River, which combine to form the Ohio River. It is the second-most populous city in Pennsylvania with a population of 302,971 at the 2020 census, while the Pittsburgh metropolitan area at over 2.43 million residents is the largest metropolitan area in both the Ohio Valley and Appalachia, the second-largest in Pennsylvania, and 28th-largest in the U.S. The greater Pittsburgh–Weirton–Steubenville combined statistical area includes parts of Ohio and West Virginia. Pittsburgh is known as "the Steel City" for its dominant role in the history of the U.S. steel industry. It developed as a vital link of the Atlantic coast and Midwest, as the mineral-rich Allegheny Mountains led to the region being contested by the French and British empires, Virginians, Whiskey Rebels, and Civil War raiders. For part of the 20th century, Pittsburgh was behind only New York City and Chicago in corporate headquarters employment; it had the most U.S. stockholders per capita. Deindustrialization in the late 20th century resulted in massive layoffs among blue-collar workers as steel and other heavy industries declined, coinciding with several Pittsburgh-based corporations moving out of the city. However, the city divested from steel and, since the 1990s, Pittsburgh has focused its energies on the healthcare, education, and technology industries. Pittsburgh is home to large medical providers, including the University of Pittsburgh Medical Center and Allegheny Health Network, as well as 68 colleges and universities, including Carnegie Mellon University and the University of Pittsburgh. The area has served as the federal agency headquarters for cyber defense, software engineering, robotics, energy research, and the nuclear navy. The city is home to ten Fortune 500 companies and seven of the largest 300 U.S. law firms. Pittsburgh is sometimes called the "City of Bridges" for its 446 bridges. Its rich industrial history left the area with renowned cultural institutions, including the Carnegie Museums of Pittsburgh, Pittsburgh Zoo & Aquarium, Phipps Conservatory and Botanical Gardens, the National Aviary, and a diverse cultural district. The city's major league professional sports teams include the Pittsburgh Steelers, Pittsburgh Penguins, and Pittsburgh Pirates. Pittsburgh is additionally where Jehovah's Witnesses traces its earliest origins, and was the host of the 2009 G20 Pittsburgh summit.