Enter "DLVTRKBWPS" as a password.
When the enemy appears in level 9, press Down. After entering the tunnel, shoot the computer and destroy the incubator. Then an alternate ending sequence will begin.
Enter "DLVTRKBBRD" as a password. Then while playing game, hold Select and press A to fly.
Enter "DLVTRKBLVS" as a password.
Enter "DLVTRKBNRG" as a password.
Enter "DLVTRKBLVL" as a password.
Enter "QVZLBVSQLV" as a password and play the game on the medium difficulty setting. Intentionally lose all lives, then re-enter the same password. Begin a new game on the easy difficulty setting, then intentionally lose all lives. Begin another new game on the easy difficulty setting, then collect the light burden. Your character will now possess the particle accelerator and the chain gun.
Note: Collect the pistol on level 2 to get ammunition for the chain gun.