At the Highway Loop level, go to the bridge with the gap in it. Go to the side opposite the long tunnel. Facing away from the bridge and to the left on the other side of the wall, you can see two round towers at what seems to be a powerplant. Hit the one closest to the bridge with a gas can. It will fall off and start rolling toward the bridge. Make sure you and no other enemies are in its path or it will break. It will slowly jump the bridge and will make a hole just inside the tunnel. You will find a black cube inside that will unlock the Elevators level in multi-player mode.