Pittsburgh, Pennsylvania, located in the United States, is the second largest city in the state and is the county seat of Allegheny County. Its population was 334,563 at the 2000 census; by 2006, it was estimated to have fallen to 312,819. By 2009, according to the Mayor's office, almost 20 percent of the city lay vacant or abandoned. The population of the seven-county metropolitan area is 2,462,571. Downtown Pittsburgh retains substantial economic influence, ranking at 25th in the nation for jobs within the urban core (and is 6th in job density). The characteristic shape of the city's downtown is a triangular tract carved by the confluence of the Allegheny and Monongahela Rivers, where the Ohio River forms. 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. In 2007, Forbes magazine named Pittsburgh the 10th cleanest city, and in 2008 Forbes listed Pittsburgh as the 13th best city for young professionals to live. The city is consistently ranked high in livability surveys. In 2007, Pittsburgh was named "America's Most Livable City" by Places Rated Almanac. Furthermore, in 2009, Pittsburgh was named most livable city in the United States and 29th-most-livable city worldwide by The Economist.