There’s a Hole in the Bottom of the Ocean

Blue holes in the deep sea lead down to flooded dungeons with better loot. About one open-sea square in forty-eight has one: a black hole in the seafloor under a cloud of dark mist. Needs Iliac Puddle No More.

There’s a Hole in the Bottom of the Ocean is an entry in the Built-in mods database, listed under On.

Several of these change core rules, so the guide pages describe the game with them on. 'Fixed by the room' means online everyone plays with the mod at its shipped settings, whatever they choose offline.