Getting the Red Key (5)

Secret Area: First off, before you even head towards the main door, there's a secret waiting. Go all the way down the current hallway. You'll see a forcefield blocking your way back to the 'Acid Pit Room'. Hit the button right next to it to drop the forcefield. Now, head back to the Acid Pit Room and dive into the acid. There's a new hole in the wall you can swim through. Emerge from there, and you'll find two Atomic Healths and two boxes of Shotgun Shells. Keep an eye out for another hidden door on the right side of the grate – the door itself is tucked away in a corner. Inside, you'll snag a case of Chaingun Cannon Ammo.

Path to the Red Key: Now, return to 'Path 01' and make your way down to ground level. You'll see another path and a door that was previously locked until you used the Blue Key. Head on in, but be careful.

Restricted Area: You'll enter a room with a window. Through it, you can spot four Pig Cops just waiting. This is the perfect spot to blast them with your RPG! Go through the door – there's a sign above it that says 'Restricted Area'. As soon as you enter, you'll run into a Pig Cop. Take him out, then head up the incline. You'll find some Shotgun Shells and, most importantly, the Red Key! Grab it, then hit the button located up there to open the big doors leading out.