Area 51

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.

Hard difficulty:
Complete campaign mode under the medium difficulty setting.

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

Alternately, successfully complete the game under the easy difficulty setting. Then, go to the last level and complete it under the medium difficulty setting. You will then have the choice to play under the hard difficulty setting. Choose to play the last level under the hard difficulty setting and complete it.
bubby693.

Hint: Dreamland: Dummies face:
In Dreamland, look at the hanging dummy. A picture of the general will be on its face.
InsideOutGaming.

Hint: Dreamland: Supply room:
On your way to pick up your scanner in Dreamland, there will be a small room with ammunition inside. If you run out, just shoot the locks on the crates. There will be more inside them.
InsideOutGaming.

Hint: Dreamland: Dummies face:
In Dreamland, look at the hanging dummy. A picture of the general will be on its face.
InsideOutGaming.

Hint: The Last Exit: Adult diapers:
When you begin The Last Exit, scan a dead Illuminati. In the category "Accessories", it will read "Adult Diapers".
InsideOutGaming.

Hint: 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.

Hint: 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.
InferNoDraGon13.

Hint: 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.
Heretic Monkey.

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.
JimmyKoz1.

Hint: Defeating Grays:
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.
Ryo Hazuki.

Hint: Defeating the Super Beast:
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.
Eryk.

When you are attacked by the Super Beast, morph into the alien. Shoot parasites and melee him until he calls the first wave of aliens. Use the BBG to kill these aliens (alternately stay in the alien form), but watch out for the Super Beast. If your health gets too low, switch to human and wait until it is safe to return to alien form. Using alien mode is safer because every time you attack the beast your health will refill.
Leeroy Jenkins.

Hint: Defeating the Theta:
To defeat the Theta easily, first make sure that you have a fully charged Mutagen bar and at least three quarters of your health remaining. There are health power-ups lying around where you fight it. Then, enter the room where he is as a mutant and either fire parasites or melee the Leapers that come after you. Using parasites on the first few to restore your health then attacking the rest with melee works well to keep your mutagen full and also leaves the health power-ups available if needed. Try not to hit the explosive barrels as they will almost certainly kill you instantly. Theta will appear after about four Leaper waves and leap at you. To defeat him easily, fire a few parasites at him then move in close to melee him. Though he is both fast and strong, in your mutant form it is very easy to hit him and dodge his attacks because of how the "mutant vision" slows him down. The only attack should worry about is his jump attack that still deals almost fatal damage, even with the damage reduction of mutant form. If you repeat this process (attacking the Leapers with melee or the parasites when they appear) you will defeat him fairly quickly and still have plenty of health and mutagen remaining for the next level.
dave brandenburg.

The best way to defeat a Theta is to first mutate. Next, fire parasites; or if you want to take a chance, fire contagion to lay some heavy damage on him. Note: Use contagion for heavy damage, but parasites also work well, especially because they heal you. When your mutagen runs low, run up and melee him and/or the enemies he releases on you (the "bubbles" or leapers). Keep doing this and he will eventually go down. This will take some time to accomplish and may be difficult at first.
christoperdawson.

If done correctly, its easier to not mutate in this battle. Save up a lot of shotgun ammunition (and dual it if at all possible). When starting this fight, jump down to the lower platform and kill the Leapers without hitting the bombs. Even if they will not damage you, save them. When Theta appears, switch to the assault rifle and wait for him to walk near to a pack of those bombs. Then, blast the bombs with a quick burst of ammo. Keep luring him, not letting him get too close, and bomb him. Continue to do this until all the bombs are gone, then switch to your shotgun(s). Run directly at him, and buckshot any Leapers on your way. Run around him, blasting him with the shotgun. If it looks like he is about to hit you and you cannot get out of the way (as one hit from him will take out basically half your health), use the secondary function of the shotgun and back up while he is dazed from the double-barrel attack. Continue running around him and fire the shotgun until he falls. Note: If you need to reload your shotgun, perform the secondary attack then back up to reload. The stumble time will give you a little extra time to refill the slow-loading shotgun.
David Kuether.

Hint: Defeating the Super Theta:
Defeating Super Theta 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.
dave brandenburg.

Only the Gray's weapons will hurt the Super Theta. You must use the BBG gun, JB grenades, and the Meson Cannon.
blaizen_1.

To defeat the Super Theta, equip the BBG. Get behind him and shoot the big green spot on his back about nineteen times, then shoot it with the Meson Cannon. He should die after all of this. Watch out for the alien amoebas he frees -- they can kill you quickly.
Starsrock706.

When you face Super Theta, you may notice he is bigger and stronger than Theta (mainly because of his cannon). Blast him a few times with dual shotguns and the shotgun special until he falls. Then go to Mutagen, run towards him, and slash away. He can still attack you, so use parasites and contagion to keep your health up. If you attack him this way, he cannot open up the grates of the bubble-like creatures. If you manage to get him down in two or three shots and are fast enough to keep him down, you can defeat him in about five minutes. Note: Contagion is the strongest. If you do not use contagion, there is a chance he will get up as it stuns enemies. Pressing all the buttons used for fighting also helps. Try to get in a rhythm so you do not lose Mutagen.
Nick Culliton.

Hint: Interesting scans:
During the training levels, scan everyone that you can. You will find some interesting equipment of the MPs, including adult diapers and condoms.
Heretic Monkey.

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.

Hint: Arcade cabinet:
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.
Heretic Monkey.

Note: Melee attack the arcade machines and you will usually get a life syringe.
Fen The Emulation Master.

Hint: Free soda:
Each time you hit a soda machine,soda's with fall out.
InsideOutGaming.

Hint: Masons and the Illuminati:
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.
Malach.