This is making big news on Yahoo! and some other major web portals: A “Secret” Subway Stop. Before you grab your flashlight and make plans to explore the unknown, prepare for a bit of disappointment.
While compelling, the news is a bit misleading and really romanticizes a relatively empty shell of a station. The City Hall station has been a well-known closed station to locals for years. Right in the heart of downtown, it is kinda hard to miss. In fact, they open the station on a regular basis for tours and they even held a gala there when the NYC Subway System turned 100 year. While a nice station, The Big Adventure would not classify it as the "most gorgeous gems in the world of Mass Transit".
Granted, it is a nice treat that you can now ride through the station as the 6 Train terminates and goes back into services, but the cars don't stop, the station is not lit up and you'll probably see more rats than archways.
Instead, we recommend bringing your camera and checking out the unique art exhibit "Life Underground" installed at the A-C-E 14th Street Station at Eighth Avenue. Much more interesting. http://www.nycsubway.org/perl/artwork_show?21
But, if you're still interesting, here is more information than you'll even need on the abandoned City Hall station: http://www.nycsubway.org/perl/stations?5:979