Manhattan is one of the five boroughs that make up New York City, and is the center of the New York metropolitan area. It is also located over the same area as a county of New York state called New York County. Although it is the smallest borough, it is the most densely populated borough and the heart of New York City. Most of the boroughs of Manhattan are on Manhattan Island, but the Marble Hill neighborhood is part of the mainland United States. Several smaller islands, including Roosevelt Island, are also part of Manhattan.