Here are three secrets to find and enjoy.

1. In the first unit go to level 3, and walk forward until you see an Enforcer, a Rebreather and a large pool area. Kill the two Light guards, then kill the Enforcer. Next grab the Rebreather and press B to activate it. Now go inside the pool area and when you see the large crack on the bottom of the pool area go in to it by ducking because you are too tall to enter it if you are not ducking. Next search around the bottom of the pool area until you see a small tunnel. Enter it and follow until you are in a sort of room of water, hold space bar until you see a little room. It will tell you "you have found a secret", and if you did it correctly you will see two bodies and a elevator. Go in the elevator and press the button. It will say "You have found a secret level"

2. After you kill The final boss in the last level, seach around the play area were you fought the boss until you see a button on the ground. There are two of them which are on either side of the play area. You should see a hole leading downwards. Enter it and you will see a button. Look straight at the button, turn around and you will see a few ammo packs and a little crack in the wall. Shoot the crack in the wall and and follow the little passage way. At the end of the short passage it says "Welcome to the id gallery". You will see pictures of the staff of id software.

3. In the Receiving center go to the area where you see a waterfall. Go backwards 5 or 6 paces and run all the way to the waterfall and then jump when you are about to jump of the ledge and land inside the waterfall.