Hard difficulty:
Successfully complete campaign mode under the medium difficulty setting.

Gray Alien in multi-player mode:
Successfully complete campaign mode under the hard difficulty setting .

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

Life Or Death: Alien grenade upgrade:
Search the dead soldier in one of the elevators where Cole starts the level to find a small key. Use it to open the small lockers in the first power conduit pit to find a databank item. It will upgrade the alien grenade so that it will track targets.

The Grays: Well armed sharks:
Proceed into the fourth room in the level, where you can see the monorail you entered the level from out of 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 strange room with sharks that have machine guns attached to their heads.

Moon Landing Conspiracy:
When you reach "Lies of the Past: Checkpoint 4", you will ascend in an elevator. Exiting the elevator, you will see a replica of the Moon's surface, a model of the U.S. space suit, and a shuttle the United States used to reach the Moon. This is in reference to the Moon Landing Conspiracy, claiming that the U.S. never actually reached the Moon, and faked the landing to beat the Russians in the space race. The conspiracy was heavily debated, but ultimately rejected by the majority of opinions. Today, only a few people still claim the Moon landing was a hoax.