Break down the wooden barricades (use a grenade if you want to be dramatic). There are several headcrabs and a bullsquid on the other side. Beyond the headcrabs is a station to the right. As you approach, a madman will break down the door. Take him out, but be careful not to hit any of the explosive containers inside. Pull the lever to call the elevator and collect supplies from the nearby room. When the elevator arrives, get on, and push the button to head down. As it reaches the lower floor, you should notice a train and a bunch of tracks. There are some medkits on the side if you need them. If you feel like going for a ride, jump on the train and throw it into full speed. Near the end of the track, the train will speed up out of control, and crash just before the slime pit, throwing you in. You can easily bypass this by simply walking around, but what fun is that?
Climb the ladder after taking care of the bullsquid here and enter the rusty pipe. Follow the pipe until it opens up. There's another bullsquid down here, so be ready for it. Walk along the pipe and jump down onto the balcony where someone was generous enough to leave out some health supplies for you. The next room can be kind of tricky, so I advise saving before attempting anything. Follow the corridor until you reach the slime pit. Walk along the pipe until you get close enough to jump onto the fallen pipe, then into the elevator. Push the button to take the elevator up and into the silo.
A New Enemy
At the top of the elevator, equip the pistol. There's a bullsquid to the right around this corner feeding on something, but you can kill it easily by shooting the small yellow explosive containers. Farther on, several houndeyes will ambush you, but since you have the pistol out, they shouldn't be a problem. From this high walkway, you will be able to see several bullsquids on lower levels. If you take them out now with the pistol, they won't be a problem later. Also, beware of the explosive barrels on various walkways. If they explode, they will destroy part of the platform. It won't cause it to be inaccessible, but it will make it harder to cross. For now, though, jump across the gap and pull the lever to open the next door.
I hope you still have your pistol out, because as soon as the door opens, a mawman jumps out at you. Further down the hall is a scientist who seems to be having trouble living. He will tell you about a creature up ahead that must be destroyed and how to do it. In the next room, you will see just what this creature is as a scientist gets grabbed by a tentacle and pulled out the window. Great. Climb up the ladder in the next room and the security guard will give you some advice on how to avoid the tentacles.
For this next part, you'll need to crouch, even on ladders, to avoid making as much noise as possible. If the tentacle hears you, it will start chopping at your position which takes almost no time to kill you. If you need to distract it, throw a grenade and wait for the explosion to make a move. I would recommend saving frequently so you don't have to go through certain parts multiple times. Toss one of your grenades out and wait for it to explode before moving out. Crawl out and down the ladder onto the 3rd floor. Crawl around the ledge to the pile of boxes with a ladder behind it. If you're quick enough, you should be able to break the boxes with your crowbar and go down the ladder before the tentacles can get to you, or if you don't want to risk it, use a grenade from a safe distance. Crawl over to the boarded up door, smash the boards and crawl through before the tentacles squash you, or better still, if you have grenades left, stand back and toss one by the door. If you made it through this part without getting hurt, take this time to give yourself a quick pat on the back.
Oxygen and Fuel
Go down this corridor and activate the airlock door. This leads back outside the silo. If you haven't destroyed this bridge yet, there will be a zombie on it just asking for death. Shoot it or the barrels beside it. If the bridge has been destroyed, you can simply jump across the pipes running beneath the bridge to get across. Follow the hall to a sign marked "Fuel Room". There are two zombies waiting to ambush you at the bottom of the ladder, so, instead of using the ladder, jump over the ledge then turn and face the zombies.
Destroy the circular grate in the floor and climb down the ladder into the sewer. Walk along the pipe, making sure not to fall off. Jump onto the upper pipe and go left. Follow the pipe to get to yet another ladder, circular vent, and ambushing zombie. There are grenades in a room on the right, so take them if you need them. Through the door are two more zombies and one giant fan.
Climb down the ladders until you're under the fan. Hit the button and climb back up the ladder before the fan starts spinning too fast. When the fan at maximum power, jump out into the void. The wind blown from the fan will push you up to the top of the room- That's one powerful fan.
Break the planks at the top and float over to the breakable duct. Knock it out with the crowbar and float in. There are several headcrabs hiding up here, so keep your crowbar out and at the ready. Near the end of the section bathed in red is a rather large breakable vent. Smash it, crawl through and down the ladder. There are a few zombies and a headcrab in this next room, so equip the crowbar or pistol before walking in. When the danger is gone, find the buttons in front of the glowing pipes labeled "Oxy On" and "Fuel On" and push them both. Once the fuel and oxygen are on, climb the ladder and open the door. Take the first right, following the red and blue pipes on the wall, until you reach the gap. Walk on the pipes if the walkway was destroyed, or across it if it wasn't, enter the airlock door, and pull the switch, returning you to the silo and the tentacles.
Back inside, you must face the tentacles again. Toss a grenade across the room to distract them, then crouch and crawl left to the next ladder. Take this down to the bottom floor and approach the next boarded up door. Smash the boards and crawl through. If you made it unharmed, save, then head to the right and jump across the gap. As you walk down this hall, debris will fall from above and destroy more of the walkway in front of you. Jump across this gap, kill the zombie feasting on a scientist, and enter activate the door labeled "02 Power Facility".
If you didn't kill the bullsquid before, he will be across the walkway. If the path is destroyed, use the power lines to get across. There are three or four houndeyes around the corner as well as a room with barnacles hanging from the ceiling and a bullsquid in the corner. When the enemies are gone, weave through the barnacle tongues and call the elevator. Get into the elevator and press the down button. Well whatever button, they all do the same thing. The elevator will break down about halfway down the shaft, so you'll have to jump to the service ladder on the far wall. Climb down to the bottom and use the pipe to stay out of the green goo. There's a scientist here and a pool of toxic sludge around the corner. If you crouch jump onto the large pipe on the wall, though, you will not get damage from it. There's one more zombie around the corner, then you're at another rather large pit. Take the cart around and climb up the ladder.
Walk around (avoiding the runaway cart) and climb the ladder on the other side to reach (finally), the switch to turn on the main power. Climb up and over (stepping over the scientist who thinks he's found the perfect hiding place) and switch on the power on the other side as well. This will activate power to the entire area. Jump down before the electrified rods extend, climb back down the ladders, and ride the cart back around.
Go back the way you came, past the scientist who will now talk about how the power's back on, and up the elevator's service ladder. There are a few mobile metal boxes here that you will need for the next part. Remember that water that you ran through on the way here? It is now electrified and running through it will take off quite a bit of health. Drag those metal boxes into the hallway and line them up over the electrified water, making a bridge. Once you're sure you can get across, run over the boxes and jump to safety. Alternative method: it is also possible to use one of these boxes to reach the top of the larger box, where you can jump onto the pipes and walk over the puddle.
Destroying the Tentacles
With the oxygen, fuel, and power ready, all you have to do now, if that scientist was right, is get back up to the booth and test fire the rocket. Run across the power cables or across the walkway if it's still there and into the airlock doors. On the other side, jump across the gaps and run down the corridor until you get to the room with the tentacles.
Well, you know what to do. Toss a grenade to distract the tentacles, then crawl toward the nearest ladder. You're going to have to get to the top, so make sure to stay crouched and use grenades when you have to. Back at the top, backtrack until you're in the room where a scientist was knocked over and grabbed by the tentacle. You should see the lights next to "Power", "Oxy", and "Fuel" lit up. If one or more of them aren't, you did something wrong or missed a switch. Check back through the guide and make sure you didn't miss a step. Otherwise, hit the button and watch the rocket do all the work for you.
When the tentacles are gone, climb down to the bottom and look down into the hole where the tentacles used to be. Climb the ladder down into the hole. Save here in case you slip and take excess falling damage. Follow the hole down and eventually you will fall into a waterhole. Climb out and look for a dead security guard next to a leaking toxic waste barrel. There will be a magnum revolver.
Swim down to the bottom and through the small circular hole. Swim to the side and up. This leads you to another room with a large pool of green goo. There's a health and HEV station at the bottom of the ladder if you need them. When you've sucked them dry, climb up to the top where you can jump onto a pipe and follow the sewers around. When one pipe dead ends, there's another to the side you can jump down to. At the end of the final pipe, jump across the gap and turn the wheel to extend the pipe across the gap. When it is fully extended, climb the ladder down into the pipe and run through it, jumping across the gap. At the end of this pipe, it will break, dropping you down into a small supply room. Gather supplies here, then proceed through the only door available into the next level.