Part 36

Swim to the surface for air, then dive to the fan. Use the "Use item" key to open the grating, then crouch to get past it and the fan.

Stay in the tunnel until drown damage reverses, then swim forward to exit.

Swim underwater to a closed gate. Climb the ladder to the right, enter a large tunnel, and proceed until section c2a5w loads.

Exit the tunnel into a four-part outdoor area. Ignore the left path with headcrabs and a health pack.

Go forward to find ammo boxes and a ladder on the left. Climb it.

From the ledge, you can jump to a supply area, but it's better to collect it later. Run along the ledge to the next ladder and climb it to higher ground.

Hide past the ladder's top and equip the gauss gun. Quicksave.

Hit the chopper with two fully powered altfire gauss blasts to destroy it.

Kill any remaining houndeyes. Go through the gap between two large rocks to the left of the electrified fence.

Enter the next outdoor area. If alerted, back up and fight marines as they approach.

Direct Assault: Equip the shotgun. Run along the left side, past a sign and two boulders. Turn left past the second boulder and double-barrel the marine. Quickly run into the dark tunnel on the right. Stop at the corner and shoot marines appearing at the entrance.

Stealth Assault (Crossbow): Turn right after entering the area. Crouch-walk along the rocky wall towards the two huge rocks. When you can see a marine's legs and arm between the rocks, quicksave and shoot him with the crossbow in zoom mode.

If the marine runs, look left for him. If he sees you, quickrestore and crouch-strafe right after shooting to avoid detection. Sneak-shoot him again and quicksave.

Crouch-walk along the outer wall until you see the feet of the marine past the second huge rock. Look left for another marine in a dark corner past two green plants. Shoot him in the head for a one-shot kill.

Crouch-walk to the second huge rock without being seen. Quicksave. Crouch-strafe slowly right until you can see part of the marine's head. Shoot him in the head.

Crouch-walk around to head-shot the final marine, who will be facing away. (Note: Due to clipping issues, you may need to use the pistol for the first two marines and they might be facing you.)

After all marines are dead, search the tunnel for supplies. Grab the marines' weapons.

Look right of the tunnel entrance for a crouch-only passage leading to health packs and AR grenades. Watch for a nearby tripmine.

In the back of the marines' area, find the doorway to a small room. Grab crossbow ammo and use the valve wheel to open a storm drain hatch.

Return to the first outdoor area. Drop onto the lower cement ledge and jump to the supply area.

Gather supplies, then run across the gap to land on the ladder. Climb back to higher ground.

Go to the rock gap you used to enter the marines' area. Look left for another rocky passage to section c2a5x.

In the next outdoor area, go around the perimeter killing headcrabs in the sand. (Note: The chopper may need to be gauss-altfire-blasted again.)