Part 16

Return to the firing chamber and go along the walkway to a room where you must hit a red switch to change which door is open, returning you to section c1a4b.

In section c1a4b, turn right after the door-switch room and shoot the explosive crate next to a bullsquid with your pistol. Finish the bullsquid if the explosion didn't kill it.

Shoot the explosives on the nearby path until they blow up, then crouch-walk across the surviving blue pipe.

On the other side, break open the two non-explosive crates for health packs, then move down the corridor until section c1a4d loads.

In section c1a4d, jump over the railing next to a ladder instead of climbing down. Run for the large floor grating ahead, bash it open, and descend the long ladder beneath it.

At the bottom, get onto the large brown pipe and move along it until it divides.

Go down the left pipe to a short ladder leading up to a bashable grating. Before bashing through, shoot the zombie below through the grating with your pistol.

After climbing up, find a small room with grenades to resupply. Move down the corridor to section c1a4e.

In section c1a4e, find the opening control for the closed door on the wall to its right. Toss a grenade at the two zombies just past the door as soon as it opens, then back up. Finish them with shotgun blasts to the head if necessary.

Move around the giant fan's upper walkway until you find a ladder to its lower walkway. Climb down another ladder to a narrow platform leading to the fan's button panel.

Quicksave, then use the button panel to turn the fan on and immediately start running backwards up the ladder to avoid being blown around or chopped up.

Jump into the space over the fan to be blown to the top of the shaft. Bash through boards with your crowbar.

Move to a large vent covering on the wall and bash it out. Toss a grenade into the vent shaft to deal with the headcrab inside, then back away.

Drop into the vent shaft and move along it. Shoot a headcrab through a large metal vent covering with your pistol before bashing it away.

Shoot the headcrab through a board barricade, then bash through it to return to section c1a4d.

In section c1a4d (again), bash through metal vent coverings and drop down next to the fuel room.

Headshot (a shot that lands on the enemy's head, usually an instant kill) the first two zombies you see with your pistol, then enter the room and blast the third zombie with the shotgun. Shoot the headcrab on top of a green machine with red lights from a distance with your pistol.

Use the "OXY ON" and "FUEL ON" buttons on the control panel left of the headcrab's machine.

Find the ladder in the alcove to the right of the headcrab's machine and climb it. Use the control on the wall to the right of the door to open it, then head down the corridor on the right to return to section c1a4b.

In section c1a4b (yet again), return to the blown-up walkway and cross it by crouch-walking along the blue pipe. Hit the switch in the door-switch room to be returned to section c1a4i.

In section c1a4i (again), go past the doorway to the rocket firing chamber to a red railing. Jump onto the railing and crouch-drop to the floor below, losing about 20 health. (Alternatively, try getting onto the left end of the railing and hugging the wall, using the Jump key or direction keys.)

Along the walkway, quicksave before a wide gap, then take a running jump across the left side.

Get out your shotgun, run forward until you reach a zombie, and double-barrel it in the head.

Enter the door-switch room past the zombie and hit the switch to return to section c1a4b.

Exit the door-switch room in section c1a4b (fourth time) to find yourself in the area with the bullsquid.