Pause the game and type "THE BEST IS YET TO BE". Press the '+' key on the keypad for near invincibility. Press '-' to for normal. If this cheat doen't work try the following cheats.
While in the pause mode, type "GROW OLD ALONG WITH ME" (include the spaces). You should now see the message {CHEAT MODE ACTIVE}. Pressing the '+' key on the numeric keypad protects your mantas and aavs from missile attacks. The '-' key can be used to turn this off. Pressing the following keys in sequence: 'HELP', 'Q', 'S', 'CTRL', and 'ALT', gives you a little message. Pressing the '9' on the keypad displays the current level. '6' and '8' have some other effect. Returning to the title screen, press the '+' and '-' keys to view all objects in the game.
While in the WALRUS arming menu, select nine Harbinger surface to surface missiles and put them in the AAV. Select the AVATAR CHEMICAL LASER and place it over the missiles. Then select the pod you want (ACCB, VIRUS BOMB, FUEL) and place it in the AAV. Then, when you've launched the AAV, look at it's weapons and you'll find you have both missiles and laser! On returning to the carrier any missiles you have left will be added to the ship's stores. Carry on doing this and hundreds of missiles can be yours.
Try pressing 'CTRL' and 'M' on the main screen for some interesting effects.