Comm Center
The sewer is the backdoor entrance to the Comm Station. The first room has a hole leading to the water below; ignore it since it only contains a grenade. The second room has a rebreather and shotgun, and is also filled with water.
The door leads to a new enemy, 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 barrels. If they are already destroyed, do a running jump instead.
Proceed through the doorway, ignoring the empty cylinder for now, and exit outside. Take a lef turn, but be careful not to enter the room; the strange boxes are explosives that can kill you, and they will detonate if a solder presses the alarm button in the corner of the room. After you kill the soldier, take the elevator up, and proceed along the catwalk to the comm room.
Defeat the enemy guards to disable the barrier, and activate the com station to get the blue key.
Returning to the installation
Head back outside, and through the locked door.
If desired, you can jump to get the envrionemnt suit, and jump into the metal cylinder. The adrenaline will restore health, and when you activate the envrionment suit, you can safely press the button to escape.
Secrets
- Near the rebreather, you may see holes in the floor in the water pit. Swim into them to eventually find an elevator leading to the Lost Station.
- Entering the elevator triggers the second map secret.