Arm yourself with a heavy duty weapon such as a Chaingun or machine gun and destroy the Enforcers, Gunners, Flyers, and Beserkers in your path. Before you enter the small passageway with the moving horizontal bars, fall in the water, swim to the surface and turn around to face the open red doorway. Climb up to this secret area and pickup the Rebreather and many health packs. The platform will take you to the very beginning of the level where you just were.
