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Cheat

When in buildings shoot paintings for grenades,etc.

Alien Mode

At the beginning of the game, shoot the first three (3) S.T.A.R.R. guys and nothing else. During this you may be shot once in a while but do not shoot back. The life comes back later and you will have alien eyesight plus your bullets look different!

Secret Rooms

It seems only proper that a secret base contains secret rooms. Here's how to find them:
Level 2: Shoot out all of the upper windows of the far back wall in the Hangar. Another can be found by shooting all the yellow barrels when you are the forklift.
Level 4: Shoot every yellow box while you're riding on the STAAR jeep
Level 5: Shoot the doorplate of the second door marked General Weatherby as the door is opening.
Level 7: Shoot all the yellow barrels at the first area where the four purple zombies are throwing barrels on the hill.

Best and Worst Endings

For the best ending, shoot off all the panels on the Mothership. For the worst ending, let the Mothership escape without hitting any panels.

Level skip

Select "Arcade" at the main menu, then aim at the black border of the boxes with "Beginning" or "Warp Ahead" to begin farther into the game then normal.

Invincibility

Pause game play, then press X, Circle(2), Triangle, Circle, X.