Area 51

Defeating Thetas
The best way to defeat a Theta is to first mutate. Next, fireparasites; or if you want to take a chance, fire contagion to laysome heavy damage on him. Note: Use contagion for heavy damage, butparasites also work well, especially because they heal you. Whenyour mutagen runs low, run up and melee him and/or the enemies hereleases on you (the "bubbles" or leapers). Keep doing this and hewill eventually go down. This will take some time to accomplish andmay be difficult at first.

Gray Alien in multi-player mode
Complete campaign mode under the hard difficulty setting.

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.

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.

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.

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.

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.

Interesting scans
During the training levels, scan everyone that you can. You willfind some interesting equipment of the MPs, including adult diapersand 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.

Hard difficulty
Complete campaign mode under the medium difficulty setting.

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

Go to a building behind the crane operations and scan a top secretfile. Go back to the secrets in the campaign menu and play DoctorCray's diary to hear him say something similar to "Man certainlyhave been to the moon but the horrors of the alien life form thatreside there have never been reviled to the public". This refers tothe theory is that there is alien life form on the moon and that thegovernment chose to hide from the public.