There are secret rooms on every level (except for Blasted Canyon). Pick a level to play at and start the game. To find these rooms, get any kind of explosive weapon (example: rocketlauncher). In every level there is some kind of writing on the walls (except for Blasted Canyon) ranging from spray paint griffiti, to posters, and to spray painted Xs. If you find one of these writings on the walls, fire a shot from your explosive weapon at it (or place a charge on the writing). After you have shot the wall, you will see that you made a hole; inside of the hole is a secret room. If you are unable to fit through the hole, blast another part of the wall off to make the hole bigger. Inside of the secret rooms you will find weapons (examples: fusion launcher, rail driver, charges, flamethrower, etc.).