Pittsburgh is the seat of Allegheny County and with a population of 305,841 is the second-largest city in the U.S. state of Pennsylvania. With a metropolitan CSA population of 2,659,937, it is the largest in both the Ohio Valley and Appalachia and the 20th-largest in the U.S. Pittsburgh is known as both "the Steel City" for its more than 300 steel-related businesses and "the City of Bridges" for its 446 bridges. The city features 30 skyscrapers, 2 inclined railways, a pre-revolutionary fortification, and the source of the Ohio at the confluence of the Monongahela and Allegheny Rivers. This vital link of the Atlantic coast and Midwest cuts through the mineral-rich Alleghenies which made the area coveted by the French and British Empires, Virginia, Whiskey Rebels, Civil War raiders and media networks.