In the level "The Grays" proceed into the 4th room in the level, where you can see the monorail you entered the level from out the window. Push the computer console there and it will say "Lifter Engaged". Make your way back outside to the monorail, you can now stand on it. Run all the way to the back of it and you will be warped to a weird room with sharks that have machine guns attached to their heads.
If you kill all 3 sharks (I used the BBG, about 7 bursts each), a wide array of goodies appears on your way back to the monorail: Health, Ammo, JB Grenades and a Meson Cannon (The Alien Rocket Launcher).
Once you return through the portal back to the bay area, do not try to get back on the monorail, or you'll be instantly killed!