Invulnerability:
Collect all of the babies in the game. A "Cheat Discovered" message will appear after the flying jetpack baby cutscene.
Secret Flea area:
In the Lower East Quack level, go past the area where you hitch a ride on the subway train. Go to the end where you see a brown fence and a little hole at the lower left side. When you are there, turn left to find a Chang transport. Once you are Chang go though the hole, and on to the boxes. Chang will start scratching himself all over. You will then be transported on his back as a flea with a heavy-machine gun.
Land Of Moles Extra Multiplayer Level:
In the first level in New Quack City, where you pushed a model building in front of a camera, there are some computer screens. One of them has the BIZZARE company logo on it. Shoot that one and it will explode and unlock the secret multiplayer level "Land Of Moles."
