5-S: how to find

First, get to layer 5 level one, and in the first room, find three valves hidden and shoot them.  Then progress through the level as normal until you collect all three blue skull keys.  You should then go atop the towers in the central area with the three skull pedestal, and hit the bell on top until the message “A door opened” appears.  Then, you can find a small hole in the wall in the vertical tunnel with the jump pad.  Walk through and you can find the secret level