In the earlier stages of the game, there are a few cabinets of the original Area 51 arcade game, complete with red and blue light-guns.

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.

Once Cole beats all the guys in the big room with the turrets, all you have to do is run straight at the turrets until you hit the gate. Then just aim up and shoot the turrets until the blow up. The turrets can't shoot back because they can't rotate far enough to aim at you.
To beat the first theta boss all you need to do is change to mutant and use your melee attack (R3) on him and the leapers. Whenever Theta or the leapers hurt you take a step back and use the leech attack (R1) on them to regain health. Then start using your melee attack (R3) again so you get the mutagen back from using the leech attacks.

Note: when the Theta steps out of the ring and sits on the side for a little while it means you have taken his health down by 1/3.

To defeat Grays that use shields, mutate and fire parasites at them. It will bring down the shields faster than any other weapon in the game.

When fighting the Super Beast, rather than running straight at him, run around in a circle. When he gets near an explosive barrel, shoot it. It does more damage then the BBG. Watch out for his plasma shooting arm as well. When you finish, Edgar will talk to you, and you get to see him.

Uses generally the same strategy as defeating Theta except for a few things. It is recommended that you ;eave at least one shot in your Meson Cannon for him because of how much damage the weapon will do. Do not worry too much if you miss, because he is still very easy. After you shoot him with the cannon, transform into mutant form and attack him with melee if your health is full or with the parasites if it is not. After a few hits he will get angry then leap up onto one of the four cryo-chambers and break it open, releasing the alien amoebas. These things must be meleed because the parasites will not trace them. Wasting the contagion on them will only drain your energy. They are not too difficult to hit in mutant form, because the "mutant vision" slows them down. Also, do not stand underneath the chamber that the Super Theta is breaking open. Afterwards he will drop almost directly underneath and will deal either fatal or nearly fatal damage when he hits the ground if you are there. However, after he does land, move in close and melee him for a few hits. The amoebas will not drop down immediately. After you kill the amoebas, return to shooting parasites and using melee against the monster. When he jumps off of the second chamber, fire your contagion at him and melee the amoebas that appear to refill your mutagen. This is done after the second one because of the sheer numbers of Black Ops that you fight before him may have severely drained your health. You may want to use the parasites to refill it. If this is not the case, feel free to use the contagion after he breaks the first chamber. After you use the contagion, repeat the process of melee and parasites until the monster dies. At the absolute longest, the fight should take about ten minutes.

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

If you start the game, only shoot the first three STAAR team members. If you do this right, you will have an infrared sight.

During the training levels, scan everyone that you can. You will find some interesting equipment of the MPs, including adult diapers and condoms. Hold right analog-stick Down (with the default controller configuration, or in the opposite direction in the inverted configuration) until your scanner stops moving and you appear to be looking at your feet. You can now scan yourself and see information about Cole.

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.

Shortly after meeting Dr. Cray, you will reach an infirmary where you will find clones of Mr. White walking around, mumbling, and rambling on. On the back of their lab coats you will find a Masonic lodge (a symbol of the Free Masons) suggesting a connection between the Masons and Illuminati.

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.

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. Go to a building behind the crane operations and scan a top secret file. Go back to the secrets in the campaign menu and play Doctor Cray's diary to hear him say something similar to "Man certainly have been to the moon but the horrors of the alien life form that reside there have never been reviled to the public". This refers to the theory is that there is alien life form on the moon and that the government chose to hide from the public.

Level 2: Shoot all the highest windows in the far wall in the hangar. When you are in the truck, shoot all yellow barrels, and you will discover another secret room.

Level 4: When you are driving on the STAAR jeep, shoot every yellow box.

Level 5: Shoot the sign which shows the words General Weatherby on the second door right away when this door opens.

Level 7: Shoot all yellow barrels in the first area where the four purple zombies are throwing barrels.

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.

Good End: Shoot all the panels of the mothership
Bad End: If you let the mothership escape

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.

Beat Campaign mode on Hard difficulty setting to get alien Gray as a selectable multi-player avatar.

Grab all the other secrets to unlock the final secret

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

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

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).