While today it is a creepy urban catacomb covered in graffiti and dirt, the abandoned 18th Street subway stop in Manhattan was actually in use up until 1948.
It was originally opened in 1904, and at the time all of the subway stations on this route could only accommodate five car trains. As service expanded, larger capacity stations were required, so the MTA began extending the length of many stations. The nearby 23rd Street station was extended southward to accommodate ten car trains, adding an exit at 22nd Street. Of course, having two stations within four blocks of each other didn't make much sense, so the 18th Street station was closed.
While this station was closed and "abandoned," it was never forgotten. Hundreds of trains pass by this station on the 4/5/6 lines, 24 hours a day, 365 days a year. It's visible from passing trains and covered in graffiti. For street artists, the station walls provide a relatively safe place to paint in an otherwise extremely busy, potentially deadly subway tunnel without many other safe places for a human and trains to co-exist. Those who have dared to enter the tunnel to paint this station, now have an audience of over a million daily commuters viewing their work.
If you want to see it for yourself, just take the 6 train at rush hour and find a window facing the wall.