To reach the secret level, go to level 3 (in the first unit) and walk forward until you see a large pool. Two Light Guards and an Enforcer patrol the area. Kill them and grab the Re-breather that they protect, activating it by pressing the B button. Now wade into the pool and slither through the large crack in floor. (You will have to duck because you're too big to enter any other way.) Once you have crawled through the crack, search for a narrow tunnel. The tunnel will lead to a tiny oasis. And once there, push the Space Bar until you discover a small "secret" room. In the room, two bodies lie next to an elevator. Step over them and push the elevator button. You'll descend to a secret level!