How does the Cistern work?

[Gamefan74]

After noticing that stuff appears in the background depending on my sanity level, I modded the game to force my sanity level down to 20 (Sanity level: ...) throughout the entire game, just to check out what's different.

The first time I went to the cistern, I turned all the valves, and left, nothing changed except for the lighting. This time around though, with Daniel being on the verge of insanity, the organic stuff and a lurker appeared for me.