The room beyond the pool and timed door should now be flooded with lava, as shown above.
If instead it looks like this—elevated walkway, 3 closed doors holding back the lava, and a gray, rocky floor—then you missed a switch in the previous area. This page shows how to go back and fix it.
Face the door opposite the entrance and take a running jump over the lava to grab the left side of the walkway (the lower of the two steps). Pull up.
Climb the two low steps then take a running jump into the doorway. Or, follow the walkway to the end, turn a little to the left, and take a standing jump into the doorway. (There's a save crystal here in NG+.)
Follow the narrow but extremely disgusting passageway to the top, jumping over a spike pit partway along.
Jump into the flooded tunnel...
...and swim down and forward through a square opening.
In the next flooded room, there are some shotgun shells (16) on the bottom and a lever below on the right (left if you're facing the entrance).
Pull the lever to open the exit on the opposite wall. Roll and grab the shells.
Then swim through the door and upward...
...to an opening where you can climb out of the water. (Of course, if you run low on air, you can return to the entrance to breathe.)
Walk forward and pick up the 2 sets of Uzi clips (17-18) on the ledge. The chomping blades in the doorway only begin to work after you pass. Once again, you find yourself in the TALL ROOM WITH LEDGES above the huge lava pool. Another flying mutant (9) approaches from above.
Rather than going back through the blades, take a running jump to grab the next ledge on the right. Pull up and pick up 2 more sets of Uzi clips (19-20).
Then kill the flier from that doorway.
Alternatively, skip the two ammo pickups at first and immediately jump over to the second ledge. Kill the flying mutant from inside the doorway, then retrieve the ammo afterwards. (You can also ignore the mutant entirely and continue, but it will be waiting for you the next time you return to the tall room.)
DARK MODE: Even with brightness adjusted, dark images on a light background can be hard to see. May I suggest a reputable dark mode browser extension, such as Dark Reader? (Download only from trusted sites, like darkreader.org or your browser's web store.)
Walkthrough text and diagrams copyright © Stellalune (email ). Feel free to copy or print this walkthrough for personal use. By all means, share it with friends, but please include this credit line so people can send me their feedback. No part of this walkthrough may be reproduced on another site without permission.