Defeating Thetas
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.


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 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.

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 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.