Crappy mod that replaces the unused Sewer Room later used in P5R (ID# 152_005) with another unused room (ID# 052_043) that softlocks the game when you try to load it normally.
To access the room, simply load Field 152005.
Have fun exploring.
That's the map it replaces, the one that loads with this mod is the larger waterfall room with the students inside the cages and the birdseye-esque camera angle.
Sorry, I should've been more specific.
Edit: It's this one