Demographics of Manhattan
New York County, coterminous with the New York City borough of Manhattan, is the most densely populated U.S. county, with a density of 70,825.6/mi2 (27,345.9/km2) as of 2013. In 1910, it reached a peak of 101,548/mi2 (39,208/km2).
Source: Wikipedia — Demographics of Manhattan (CC BY-SA 4.0)