To the left of City Hall can be seen the entrances to the original City Hall subway station , which closed in 1945. I was lucky to join a NY Transit Museum tour of this magnificent space many years ago and highly recommend it. If you can't get tickets for the organized tour (they sell out very quickly), here's a way you can at least get a quick glance: if you stay on the downtown number 6 train past the Brooklyn Bridge terminus, the train uses the abandoned station to turn around and head back uptown. If the lights in the abandoned loop station are on, it's a great view.

This is the location of City Hall Station. A beautifully designed and decorated subway station that used to be part of the 6 (East Side) line. See https://www.nycsubway.org/wiki/IRT_East_Side_Line and especially https://www.nycsubway.org/wiki/Station:_City_Hall_(IRT_East_Side_Line)

The two entrance kiosks are no longer present, having been removed and the street penetrations paved over. Shown at the bottom of the photo is a grid of small squares. These are glass bricks, acting as a skylight over the station.

A tour is available to members of the New York Transit Museum.