You're now midway through the timed run begun in the previous section. Lara has just turned valve #3 and dropped into the flooded tunnel. (Or, if you aren't after the "Super Lario" achievement/trophy, you're midway through completing this sequence at a relaxed pace.)
Dive below the surface (Jump + Forward). Then follow the long pipe straight down.
At the bottom, swim into the perpendicular pipe, and follow it away from the grate-covered opening.
At the next intersection, there's another grate on the left. Turn right (the only possible path).
Continue straight to the next intersection. Another grate blocks the way ahead, so turn upwards...
...and swim toward the surface. (Again, this is the only possible route.)
As you come to the surface, head to the right and aim for the space between the collapsed catwalk and the dark metallic pipe.
Use the collapsed catwalk as a ladder to climb out of the pool.
Follow the walkway to a break in the railing. Jump straight up to grab the narrow, horizontal pipe...
...then climb out to the middle of the room. Drop onto the black-and-yellow crossbeam. Turn left, walk/run to the end of the beam...
...and turn valve #4.
The third light from the right on the valve-monitoring panel turns green.
The ladder near the valve leads nowhere, so return to the middle of the crossbeam and jump up to grab the pipe. This time, traverse all the way across and drop onto the catwalk.
Climb the bent catwalk to get up top.
Follow the walkway around the room counterclockwise. Jump the gap where the walkway is broken. Run to valve #5 and turn it.
The second light from the left on the valve-monitoring panel turns green. Just one more to go!
NOTE: There's no need to go into the huge pipe near the gap in the walkway. It's possible to crawl under the bars and slide down the chute, but this dumps you way back near the fan where you got the strength upgrade. Then you'll have to swim through the pipe and climb all the way back up here.
Return along the catwalk, jumping the gap in the other direction. Walk to the edge of the bent section where you climbed up earlier.
Jump forward and grab the next section of walkway on this same level; pull up. Go through the door just ahead.
Follow the hallway, picking up M-V9 ammo (6) on the way (if there's time). You emerge back at the top of the first hub.
Walk out onto the black-and-yellow crossbeam. Stand under the narrow pipe running across the shaft. Jump up to grab it, traverse all the way across, and drop onto the small ledge near valve #6. Turn it.
The camera pans to the right, showing that water is no longer gushing from the huge pipe. And, if you've been fast enough, the "Super Lario" achievement/trophy unlocks. Wahoo!
Jump up and grab the narrow pipe once more. Climb hand over hand along the pipe and drop down onto the black-and-yellow beam.
NOTE: If you missed the M-V9 ammo (6) in your haste to reach the last valve, go back into the hallway on the right to get it. There are no more pickups ahead, but you can re-explore this area for anything you may have missed. (statistics) Once you enter the next area, you can't return here.
When you're ready to move on, follow the crossbeam to the far end and jump into the huge pipe that was previously blocked by flowing water. Climb into the smaller pipe within, open the door, and enter a new area.
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