Tut... this map again... oh well here are the secrets:

Vent Secret 1 - The Head Crab: Well this one is the most known one out of the secrets i guess... It's where you press a button to destroy a vent and then you can get behind a headcrab to get invincibility (the other side of the headcrab kills people). To open this secret you must go into the vent go right and follow it round until you get to a 2 way choice. Go left at the 2 way choice and press use (e by default) on the secret button that is on the wall directly infront of you:

This destroys this grate that is hardly visible on my picture:

After about 30 seconds to 1 minute the grate will break allowing you to get to the headcrab:

Vent Secret 2 - Outside the map: (Use invinicibility to do this part!)

Infront of the headcrab there is another secret button that breaks the vent grate infront of the headcrab (the grate that i am looking at). Press use on the button:

Follow the vent around after the grate has broken... You will drop down a small fall. Once you fall down it turn around, there is another button on the wall. Press use on it.

This breaks this grate and you can get outside of the main map, it is also the starting picture you see whilst joining the game:

Chicken Secret 3: This secret involves the top detonator. Once you take out the 'sniper' wait at the top detonator and keep pressing use on it until there are about 4 - 5 explosions. Once you go to where the 'sniper' was there will be a doorway where a door used to be. Around the corner is a chicken and text comes up on screen saying: "Noone here but us chickens  :)".

Alien Secret 4: The fourth secret is after the acid and barrels up the ladder and it is the second door on your left. The bottom of the door is breakable and then there is a breakable box inside the half broken door. Hit the small alien to make it do different things.

That concludes K_Thegame2_r.