Mystery Incline at 42nd Street Between First and Second Avenues

Notice the 2nd Avenue El to the left of the frame (from 1924 aerial photograph) and the incline at 42nd Street between First and Second avenues..  Current street elevations are shown imposed on the 1924 aerial photograph.  Notice that the street elevations do not change until you appoach Tudor City Place.  Actually, the street elevations are increasing as you go east then drop off fast near the river.  You can clearly see the shadow of the incline.  I looked at aerial phoographs beyond the frame of the picture and I do not know where the tunnel leads to.  Thanks threestationsquare for the info. [ The United Nations complex is off the map and is north and east of the diagram.]  Tramway Null (0)