After navigating the four pits, as shown in the previous section, continue along the passageway, shooting 2 rats (4-5) on the way. (Strange how Lara can subdue a big burly dude with her fists but won't stomp a tiny rodent.)
Pull the large chunk of rubble to shift the pile blocking the passageway. You still can't get through there, but it opens the way on the other side.
Kick open the grating nearby. Crawl through to the next room...
...and pick up some Vector ammo (1). Go through the door...
...then a second door, to return to where you started.
(The first pit is off to the right, and you may see the first rat you killed if you haven't reloaded.) To the left is the same pile of rubble, only now you can get past it.
Proceed to another flooded pit. There's no way to avoid this one (and it doesn't affect the achievement/trophy), so jump in, swim across (use Jump plus directions to swim), and climb out on the far side (Action + Forward).
Enter a room with a bright light and some pipes. Make a U-turn to the right and exit through the hole in the wall...
...into a storage area with a barred door and a few coffins. Go through the doorway on the left.
In the next room you'll find a man covered in some hideous, metallic goo squirming on a cot. Next to him is a small medipack (2).
Exit through the open cell door. Turn left and proceed through the metal door with the rivets at the end of the corridor.
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