Area 51

Right before you are about to experience an in game cut scene push Up on the D-pad to change into the mutant form and then activate the cut scene to view the cut scene in mutant vision.

Find and scan all five databank items in a level to unlock the secret of that level.The final secret of the game will be unlocked when all prior secrets have been found.

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.

Grab all the other secrets to unlock the final 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.