This sounds like a very interesting world. In my homebrew, I had a cosmology where there was a plane with no surface, it was what folks from the material world would recognize as "subterranean".
Except I had small-continent-sized caverns, not just large rooms, or arena-sized chambers. Large enough to get circulation of air and maybe even something like a weather system for these immense caverns. There would still be standard caves.
I hand waved things like air pressure at elevations, or water pressure at depths. Said it was all the same pressure regardless of height or depth.
At a certain base elevation, all the water had seemed to pool up. Call it a standard sea level, but water will cycle into the atmosphere, mostly by evapotranspiration, but thermal activity in certain geologic hot spots warm up the water into gas where it cycled back to the air, rising up through the cracks in the earth as high as it could and then come back down.
I imagined miles tall stalactites, stalagmites, and columns with settlements clinging to the sides above the miles deep sea below in some of the continent-caverns. Use of aerial travel between place to place, except when exploring deep into a side or ceiling tunnel or rift.
Cyclical phosphorescent plant life clinging to cavern walls could offer a circadian rhythm to life below.
I never got to run adventures in it, but all the maps I sketched out years ago are still in my shelf waiting to sea use someday!