Area 51 Cheat Codes for XBox

Unlock Final Secret
Grab all the other secrets to unlock the final secret (page 5). Well, d'uh.

Unlocking Secret Tapes
Scanning all five databank items in each stage yields one secret. Completing the game yields one secret.

Upgrading The Alien Grenade
In the stage Life or Death, there is a small key on a dead soldier in one of the lifts where Cole starts the stage. Grab it to open the small lockers in the stage. One of the small lockers in the first power conduit pit has a databank item that upgrades the alien grenade (it now homes in on enemies).

Sharks with machine gun heads
In the level "The Grays" proceed into the 4th room in the level, where you can see the monorail you entered the level from out the window. Push the computer console there and it will say "Lifter activated". Make your way back outside to the monorail, you can now stand on it. Run all the way to the back of it and you will be warped to a weird room with sharks that have machine guns attached to their heads.

Unlock the Alien Multi-player Avatar
Beat the Campaign mode on the Hard difficulty setting to unlock an alien Gray as a selectable multi-player avatar.

AREA 51 arcade game
On the HOT ZONE mission at the beginning of the level.Hidden in a corner there is the original AREA 51 arcade game complete with red and blue lasar-light guns.