Manhattan is the oldest and the most densely populated of the five boroughs of New York City. Located primarily on the island of Manhattan at the mouth of the Hudson River, the boundaries of the borough are identical to those of New York County, an original county of the state of New York. The borough and county consist of Manhattan Island and several small adjacent islands: Roosevelt Island, Randall's Island, Wards Island, Governors Island, Liberty Island, part of Ellis Island, Mill Rock, and U Thant Island; as well as Marble Hill, a very small area on the mainland bordering the Bronx. The original city of New York began at the southern end of Manhattan, expanded northwards, and then between 1874 and 1898, annexed land from surrounding counties. The County of New York is the most densely populated county in the United States, and one of the most densely populated areas in the world, with a 2008 population of 1,634,795 living in a land area of 22.96 square miles (59.5 km2), or 71,201 residents per square mile (27,485/km²). It is also one of the wealthiest counties in the United States, with a 2005 personal income per capita above $100,000.Manhattan is the third-largest of New York's five boroughs in population, and its smallest borough in size.