When you defeat the huge mutant, the exit door at the back of the platform opens. Go through.
Follow the fleshy passageway forward, then left. Slide down the ramp into a gray-tiled hallway.
Pass the movable block in the alcove and continue down the next ramp. (The sound of chomping blades is audible from somewhere above.)
Soon you'll come to another movable block. Push it 3 times.
Go up the ramp on the right...
...and around the corner to reach the back side of the first movable block you passed earlier. Push it once.
Then go around the right side of the block and climb on top of it.
Turn around and take a standing jump to grab the ledge ahead. Pull up into a hallway with a break-away tile (43/51) concealing a spike pit, with chomping blades beyond. (NG+ players get a save crystal here.)
Stand at the edge of the solid floor just before the cracked tile and watch/listen to the blades. Start to run through the blades on the second "clang," or just as they begin to open. Continue forward and around the corner.
NOTE: Shattering this break-away-tile contributes to the total for the "After Us, The Deluge" achievement/trophy. Just 8 more tiles to go.
Don't go down the next ramp. Instead turn right into a corridor with light and dark tiles.
Follow it to the bottom...
...and push the movable block once.
Return to the top of the passageway, turn right, and go down the ramp you didn't take earlier.
At the bottom is the block you just pushed, a lava pool on the left, and a red door and switch that are too high to reach.
Climb over the movable block.
Then push it twice, so it ends up beneath the switch.
Follow the passageway back up and around until you're standing on top of the block you just moved.
Throw the switch to open the red door.
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.)
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