There is a small part of Manhattan that is physically part of the Bronx borough to the north. It is the neighborhood of Marble Hill. The two boroughs have been playing tug of war over this former island neighborhood for over a century.
The quarrel started with the building of the Harlem Ship Canal, which cut right through Manhattan's northernmost neighborhood, turning Marble Hill into an island. During 1914 the canal was filled in, making Marble Hill now physically part of the Bronx, but still legally part of Manhattan.