Unlockables
Hard difficulty.

Complete the game on normal difficulty to unlock the hard difficulty.


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.


Easter Eggs
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.


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.


Glitches
Reduce Damage When Falling From A High Place

As soon as you are about to fall, press the crouch button to reduce damage.


Secrets
Alien grenade upgrade.

In the life or death stage there is a small key on a dead soldier in the damage elevator where cole starts the level.Grab the key and open all the lockers in the level.One of the small lockers in the first power canduit pit has a databank item that upgrades the alien grenade.The grenade will now track it's target.


Secret tapes.

Find and scan all five databank items in a level to unlock the secret of that level.The final secret of the game is unlocked when all prior secrets have been found.