So Chris Kutalik asked for an underworld layer
here. And I REALLY REALLY need to be working on something else, but then I figures “better to get it out of my brain and down on paper instead,” so here it is.
Disclaimer: Where I say “Ark” and “Noah” please substitute your own fun – Baldur would be fine, or Dave Bowman or Turkmenbashi or whatever you like. Even Noah, I guess. I kinda like the conceit of the Other Ark, with all the creatures that we don’t see around us.
The ark project – a multi-level “open menagerie” – was either a miserable failure or too successful. Point is, there was never a good moment or method for letting the animals out – things got pretty hairy in there after the second generation of forced evolution. So it was sealed up tight behind a combination lock, the key to which was buried with Noah, and a complicated system was made for controlled release of the animals or for drowning the project altogether… one day.
Millenia passed, the complex fell into ruins, and a few critters have escaped – they roost in the ruined machine halls (blue on the map) around the Ark itself. Now those escaped beasties make the whole place dangerous – especially around the Great Lock in the old entrance hall, which links multiple floors in a big open court. The smarter beasts know what will happen if that door is opened, so they keep a watchful eye on the Lock and try to stop anyone meddling with it.
The machine halls can be flooded individually or the whole complex can be flooded, using the still-working pump houses (green on the map) – to neutralize the threats of beasties in one set of halls or another. Some halls get soak occasionally by malfunctions, though: they contain sea life (giant anemones, nautili and such) that’s dormant unless you flood them back to life. If the whole complex is flooded, folks up on the surface will first notice the level of the river sinking abruptly, then once it’s all flooded, the central Ark piece will break loose and rise up to become a new island in the river.
Treasure can include ancient tech tools and gewgaws (eg “jewels” that are really light-up plastic buttons), knowledge (especially about ancient species/maps of long long ago) and the critters, if they can be captured.
Each of the machine halls is an extensive dungeon complex, as is the Ark itself. Pump houses consist of only a few rooms and are uniformly some stories above the galleries: A is particularly inaccessible: reaching it from any direction involves an arduous climb.