The skyline of Pennsylvania's largest city, Philadelphia. Visible are Center City with the Comcast Center on the left bank of the Schuylkill River and the 30th Street Station with the Cira Centre on the right. Philadelphia was once the largest city in the original 13 American colonies, the location of the signing of the Declaration of Independence, and served as one of the nation's many capitals during the Revolutionary War and after.Photo: Massimo Catarinella