Use the switch to open the airlock, then step inside. Those small canisters you see lying around can put some hurt into an alien if you shoot them, but they can put some hurt into a walkway too, so be careful. Travel around the outside edge of the big deep pit, killing monsters along the way. Run quickly over the the slanted pipe, run up it onto the big lift, then press the button to start the long journey upwards.
Go down and to the left, then jump off into the slime. Run on top of the pipes around and to the right, then jump off, over the rail, back onto solid ground.
Jump out of the pipe again, and go to the left to climb up a ladder. Climb up the ladder on the other side and drop down into the big open pipe up there.
Hop up on top of the now-stopped train to reach the ledge which circles around the slime pit. Olympic hopefuls: practice your Greg Louganis impression by seeing how far you can launch into the slime pit.
