Legend: (Youtube), (Forums), (Trophies/Achievements), (Controls), (Savegames), (Gear), (Traps), (Enemies)
The DrainRun ahead. On the right side is a rat. Shoot it or just avoid it to save ammo. On the left side is a Chocolate Bar. Run up the nearby stairs to find a Large Medipack at the top. Run back down and walk through the water. In the next tunnel is another rat. On the left is a passage leading away, we will come to that later. At the end you will come to a chamber with a huge drain in the middle. Avoid it, dropping down will kill you. A couple of pipes lead away from here. On the left is a control panel with 6 red dots. Your first task is fulfilled when they are all green.
Drainpipe to the TopSearch for the drainpipe on the opposite wall of the shaft. Lara can climb up. At the top shimmy to the left and pull up onto the walkway. Run to the end where you can use a switch to stop a fan somewhere else. Climb back down.
Fan & First ValveRun into the tunnel where the bigger pipe comes out. On the right you will find an alcove where a ladder leads up to a fan. Climb up. Position in front of the fan and press action when the Hand Symbol appears to turn the fan a bit. Power Up (upper body strength) Crawl through the gap and collect the V-Packer Ammo on the other side. Run ahead and in the next chamber use the valve. In the cutscene you can see that the first light turns green, five more to go. Return the way you came to get back to the drain.
Second ValveRun back into the passage where you first came out. That's the one to your left, the round tube. Turn into the passage on the right. Dive along the passage. At the end you will come to a chamber with a ladder. Climb up and kill the rat. This one here is an annoying beast. Turn the next Valve you find up here. The second light from the right goes out. Turn it. Then run out into the passage and collect the Ammo. Step through the door. If you run through the shaft towards the right you will come back to the deep drain.
Third ValveUse the drainpipe to climb up again. Follow the path till you reach the walkway that runs across the room to the opposite side. Walk (!) over it to the other side. There you will find another valve you have to turn. Collect the Ammo at the right end of the walkway.
Fourth ValveIn the corner you can see a ladder overhead. Jump up and hold on to the ladder. Climb up and leave the ladder to the left. Further to the left you will find the next ladder you have to climb. You can leave this ladder in both directions. Run to the right to find a bar overhead on which you can monkey climb to the other side. On the other side you will find the next valve, which you have to turn. Use the bar to climb back to the other side. Halfway over you can drop onto the walkway spanning across the room, then you can reach the red-brown passage but we will come to that later. Climb back down to ground level. If you are daring and good in aiming a jump you can jump into the drain in the middle, with 4 valves turned it is now filled with water.
Fifth ValveDive down into the deep shaft in the middle. Follow the tunnel. There you have to follow the tunnel to the right and then at the end dive up to the surface. Here you can find a grating you can climb up. On top you run ahead till you come to a bar, where Lara can climb over to the other side. Climb along till you are above the narrow walkway in the middle. Drop onto it. Go to the left end, where the next valve is. Use it.
Sixth ValveReturn to the walkway in the middle of the shaft. From there jump back up to the bar again. Climb to the side where you have not been yet. There you will find a very deformed grating which you can climb to the top. Go to the end of the walkway and jump over the gap to reach the last valve. Use this one as well.
Clear PassageTake a running jump towards the far walkway with the door. This is the jump Lara was referring to when saying she can make it. Step through the door to get into the red-brown passage from the other side.
Another one. On my second time through I pressed the action button when the symbol appeared and Lara went straight through the closed door and picked up the Ammo.
Now you can choose between two paths. Path a or Path B.
a. Explosion on the Lower LevelHang from the ledge. Drop down onto the walkway. Run along to the left. You will come to a boiler. Lara will tell you that she thinks she has found the right spot. Place an explosive at the boiler and then backflip to land in the water. You cannot dive up towards the surface since the oil has caught fire. Dive into the tunnel instead. At the second turn swim up to the left. Here you can catch some breath again. Swim up till you can get out of the water. Run up till some pipes are blocking your way. Turn left. At the end you have to turn left again. Drop down onto the walkway below. Run to the right till you reach the ladder. Climb up and down again on the other side. Make a backflip from the ladder to get over the burning part. Then climb up the ladder at the right wall and leave it onto the walkway on the left. From the end you can jump down towards the hole in the wall. Enter. Run to the left till you reach the green lit gate.
Some people reported that they cannot get through the wall. I guess there might be an invisible wall. So far I have found no solution or what's causing it. Just be careful that you do not take any unnecessary detours and save often to different slots, so you can try again.
or b. Explosion on the Upper LevelDrop down onto the walkway. Run to the left and climb up the ladder onto the elevation. On the other side you climb down again. Run around the corner and past the boiler till you reach a ladder. Climb up. Leave the ladder to the left. Then turn around and take a running jump towards the pair of boilers on the platform over there. Lara thinks she has reached her destination. Place an explosive at one of the boilers. Jump quickly into the water. You cannot dive up towards the surface since the oil has caught fire. Dive into the tunnel instead. At the second turn swim up to the left. Here you can catch some breath again. Swim up till you can get out of the water. Run up till some pipes are blocking your way. Turn left. At the end you have to turn left again. Drop down onto the walkway below. Run towards the elevation with the broken ladder. You can climb up without ladder. Climb onto the broken boiler you find up here. From here jump up to the pipe at the ceiling and monkey climb ahead to the opposite side. Drop onto the walkway there. Run up to the top. There you can walk over the tilted drainpipe and towards the opening in the wall. Drop down into the passage on the other side. Run to the left till you reach the green lit door.
Again you might encounter the bug mentioned in part a.