Pittsburgh is the second largest city in the U.S. Commonwealth of Pennsylvania and the county seat of Allegheny County. Regionally, it anchors the largest urban area of Appalachia and the Ohio River Valley, and nationally, it is the 22nd largest urban area in the United States. The population of the city in 2010 was 305,704, while that of the seven-county metropolitan area stood at 2,356,285. Downtown Pittsburgh retains substantial economic influence, ranking at 25th in the nation for jobs within the urban core and 6th in job density. The characteristic shape of Pittsburgh's central business district is a triangular tract carved by the confluence of the Allegheny and Monongahela rivers, which form the Ohio River. The city features 151 high-rise buildings, 446 bridges, two inclined railways, and a pre-revolutionary fortification. Pittsburgh is known colloquially as "the City of Bridges" and "the Steel City" for its many bridges and former steel manufacturing base.While the city is historically known for its steel industry, today its economy is largely based on healthcare, education, technology, robotics, and financial services. The downturn of the steel industry left no steel mills within the City of Pittsburgh and only two remaining mills in the county, though more than 300 steel-related businesses remain in the area. By contrast, the region supports 1,600 technology companies, ranging from a Google campus to small startups. The city has redeveloped abandoned industrial sites with new housing, shopping and offices, such as SouthSide Works and Bakery Square.