Who lives there ?
- Vampires, seeking relief from the surface sun. No need to breathe : they are dead, remember ?
- Crab and lobster people. They are working on reversed diving-suits that would help them conquer the surface.
- A sleeping elder god and its faithful cultists. Of course, its awakening is getting closer and closer.
- Water Giants, and their shark pets. Food is getting scarcer and scarcer and they plan to come and get it on the surface..
- Wood elves. They protect a magic seed that will allow to reforest the surface land.
- Highly dangerous prisonners and their guardians. An insurrection is coming.
What's around the city ?
- A field of precious coral. The inhabitants sell it to finance the maintenance of the city.
- Ghosts of drowned sailors. They are convinced the citizens are responsible for their tragic fate.
- A terrible kraken. The inhabitants possess a secret that had allowed them to repel him. Until now...
- The emperor's army in magical bubbles that allow them to breathe. They lay siege to the city.
- Mind controlling anemones. They are the ones pulling the strings in the city.
- Magmatic pipes. They provide relatively safe energy to the town.
What's on the surface ?
- A huge gathering storm. Some says the inhabitants are responsible for it.
- An arid desert. The city is located in an oasis.
- Techno-hell. This body of water is the only place in the area where life still exists.
- Ice floe. It acts as a barricade protecting the inhabitants.
- A peaceful village. It suffers from the exactions of the city denizens.
- An old monastery. The priests are watchmen and diplomats, monitoring and interacting with the inhabitants of the city.
To create my generators, I take a lot of inspiration from books like Tome of Adventure Design, Artifices Deceptions and Dilemnas or the Monster Overhaul. You should really take a look if you are into world building. Also here's a GREAT submarine adventure by the Merciless Merchants : Ascent of the Leviathan.