Underground, New York City is crisscrossed with a number of abandoned tunnels. And a few times a year, the public is allowed to explore them, legally.
About once a month, the half-mile Atlantic Avenue Tunnel opens up for tours led by the Brooklyn Historic Railway Association. The tunnel, built for a train in 1844, is opened up from Court to Hicks streets and is reached through a manhole in the street near Trader Joe's market.
The winter schedule of the New York City Transit Museum tours also offer access to abandoned spaces.
On March 7, it will host a tour of the original City Hall subway station, which was opened in 1904. You can still see its skylights, chandeliers, and arched Guastavino ceiling tiles. Reservations are required and it usually sells out early.