Episode 1: To get to the Launch Facility you must go to the last lighted area before the exit in the Toxic Dump level, you will see a cracked wall that hides the secret level exit.

Episode 2: In Occupied Territory when you are half way accross the bridge to the exit, turn and look back at the control booth. There is a switch high up on the column to the right of the booth. Shoot the switch to open the secret area left of the bridge when you face the control booth.

Secret Level 2: In the Dark Side, well on the other side of the obelisk, there is a cracked wall. Inside is a passage to the Lunatic Fringe secret level.

Episode 3: In the Movie Set next to the yellow key card and a sattelite there is a USA sign. Push the sign to access the secret level Tier Drops.

Secret Level 2: In Hotel Hell there is a waterfall that hides a teleporter. Get in the telaporter and blow away the flaming pots to get to Freeway.