Northfield Mountain pumped storage hydro plant contains an underground generating station blasted out of gneiss. The powerhouse is accessible through a 2,500-foot-long tunnel. Seven hundred feet below the surface, the cavern is longer than a football field and higher than a ten-story building.
Tunnels, Power - Hydro
The image shows the service road entering the only access to the tunnel. The underground powerhouse flooded during construction in 1972 and suffered a fire in 2002: http://www.masslive.com/news/parchives/f619tunn.html