On the stadium level, look up and you'll see a blimp that says "Duf Beer." If you shoot it with your missile launcher, it will explode and lots of ammo, health packs, and other helpful items will fall onto the field.
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.
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.
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.
2. Also in the projector room press the button t open doors on the wall next to the fire extinguisher. It will open and you can look around and see what's inside.
3. Go back downstairs where there is a cash register behind a bar. Click on the cash register and you SHOULD see a door open off to your right. There are two ways to get it, either use your jetpack to fly up or search around for a little narrow elevator to take you up.
4. Go back outside to the innocent? sign and jump on the box. If you jump staright, you should land on a ledge. Keep trying to jump through the windows and on the third or fourth one you can go inside a room.
5. Also in the room there is a poster that says attack of the bleached blond biker bibmbos. open it and there's another secret.
6. Outside by the cinema sign turn on your jetpack and fly up to the glowing red sign. There is a row of black windows. try going through each one until u fall into a room.
7. This doesn't count as a secret but when you walk to the area where you beat the level, turn on your jetpack and fly up as high as you can. There will be two guns shooting at you. One gun is in a small room and in the room is a jetpack and bullets.