Concrete is the most widely used man-made material in the world. It is hidden under the surface of the city: forming the foundations of skyscrapers, snaking underground as tunnels, retaining tons of earth laterally to clear space for inhabitation, or anchoring the steel structure of the elevated train lines. New York is, after all, the concrete jungle.

Crew waterproofing reinforced concrete entrance to the 60th Street Tunnel, October 17, 1917. Plans originally called for the BMT Broadway Line to connect to Queens lines via the Queensboro Bridge, but this plan was scrapped due to uncertainty over the bridge’s ability to support additional train and vehicular traffic. The tunnel opened on August 1, 1920, in an event timed to coincide with the opening of the Montague Street Tunnel; together, the tunnels connected the Manhattan’s BMT Broadway Line to service in Brooklyn and Queens.