Part 36

Surface for air, then dive and swim to a fan area.

Lean on the "Use item" key until the wheel stops, then move into the fan area past the grating.

Crouch to get past the grating and fan, then enter a tunnel leading to the other side of the dam.

Stay in the tunnel above water until drown damage reverses, then swim forward to leave.

Swim underwater to a closed gate, then climb the ladder on the right wall.

Enter a large tunnel and proceed until section c2a5w loads.

Exit the tunnel into a four-part outdoor area. Go forward from the pipe exit to find boxes for ammo and a ladder on the left.

Climb the ladder and run along the ledge to the next ladder, which leads to higher ground.

Hide past the top of the ladder, equip the gauss gun, quicksave, and hit the chopper with two fully powered altfire gauss blasts.

Kill any remaining houndeye, then go through the gap between two large rocks to the left of an electrified fence.

Enter an outdoor area with hidden marines. If alerted, back up and take them down as they advance.

Alternative Marine Assault (Shotgun): Get out the shotgun, run along the left side of the area, passing a sign and two large boulders. Turn left past the second boulder, double-barrel the marine hiding there, then quickly run into the nearby dark tunnel. Stay on the right side and stop at a corner to shoot marines appearing at the entrance.

Alternative Marine Assault (Crossbow - Sneaky): 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, zoom, and shoot him. If he runs, look left for him. If he sees you, quickrestore and crouch-strafe right after shooting. Sneak-shoot him again, quicksave. Crouch-walk along the outer wall until you can see the feet of the marine past the second huge rock. Look left for another marine in a dark corner past two short green plants and head-shot him. Crouch-walk to the second huge rock without being seen, quicksave. Slowly crouch-strafe right until you can see part of the marine's head, then head-shot him. Crouch-walk around to head-shot the final marine who is facing away. (Note: Due to clipping issues, you may need to sneak-shoot the first two marines with a pistol.)

Search the tunnel for supplies, then 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 a doorway to a small room. Grab crossbow ammo and use the valve wheel to open a storm drain hatch in another area.

Return to the first outdoor area. Drop onto the lower cement ledge, then jump to the supply area skipped earlier.

Run across the gap to land on the ladder, then climb back to higher ground.

Go to the rock gap you used to reach the marines' area, then look left for another rocky passage to section c2a5x.

In section c2a5x, avoid the tentacle beast (it cannot be killed) and collect ammo near it.

Go around the perimeter killing headcrabs in the sand. (Note: The source text suggests an alternative start for c2a5x: gauss-altfire-blast the chopper again.)