Quake II/Comm Center

Comm Center
The sewer is the rear entrance to the Comm Center. The first room has a hole leading to the water below; ignore it, since it only contains a grenade. The second room has an Underwater Breather and shotgun, and is also filled with water.

The door leads to a new enemy, a Parasite, which will appear from the darkened ramp from the floor. To continue, climb up the stairs. To get the Quad Damage, climb onto the nearby crate and jump across the explosive radioactive containers. If they are already destroyed, execute a running jump instead.

Proceed through the doorway, ignoring the empty cylindrical tank for now, and exit outside. Take a left turn, but be careful not to enter the room; the strange boxes are explosives that can kill you, and they will detonate if a Enforcer presses the alarm button in the corner of the room. After you kill the Enforcer, take the elevator up, and proceed along the catwalk to the communications room.

Defeat the enemy guards to disable the barrier, and activate the communications station to get the blue key.

Returning to Installation
Head back outside, and through the locked door, now opened by the blue key.

If desired, you can jump to get the Enviro-Suit, and then jump onto the catwalk at the bottom of the cylindrical tank. The adrenaline found at the bottom will restore health, and when you activate the Enviro-Suit, you can safely press the button to escape. The tank fills to the brim with acid; swim up and step out.

Secrets

 * 1) Near the Underwater Breather, you may see holes in the floor of the water pool.  Swim into them to eventually find an elevator leading to the ../Lost Station/.  By diving directly into the correct hole in the bottom of the pool, it is possible to swim the distance without using the Underwater Breather.
 * 2) Entering the elevator to the Lost Station triggers the second map secret.