[Here's some writing. It will explain some aspects of the abyss in a way I can't otherwise.]
"It's beautiful", uttered Ivan, in a rare moment of appreciation. "Rocks and platforms and chunks of land all floating in an endless abyss of the blackest black. This one's different from the others. If we jumped off this ledge, we wouldn't fall too far."
The lithomancer, Tumor, glances up.
"The air here is breathable, but so thick you can almost swim in it. Seeing as how you're a pyromancer, Ivan, you could probably propel yourself regardless; however, Rhys and I will need to take advantage of its odd thickness."
Rhys jumps into the abyss. She travels a very short distance before halting in the air. She makes a breaststroke motion and moves forward a bit.
"It's amazing, I'm completely weightless. Come on in, the chasm's fine."
The other two jump into the rift and begin trekking across.
The abyss is deep underground but not deep enough to reach the planet's (presumably hollow were this the case) core. A waterfall on a distant floating mountain evaporates into the abyss, filling it with clouds. Occasionally, movement is seen. Never is wind felt, however. Perhaps they're not alone?
So, how can I make this theoretical abyss possible?
Some details and elaborations about the chasm from the story:
- It's not a little chunk of space, as air is not only breathable, but thick enough for reasonable propulsion.
- Nor is it a hollowed core at the center of the planet.
- It's big enough for weather and huge chunks of land resembling small mountains to be dwarfed by its vastness.
- As it's an underground chasm, there's no wind - the air is still.
- No, by "the air is thick" I do not mean the cavern is flooded with water. Things like clouds and pyromancy would be rather difficult in that situation, not to mention any buoyant force on the landmasses and such. It means that the air resistance is high enough to give a bow an effective range of no more than about 40ft and allow for something similar to swimming to propulse oneself.
- Bioluminescent molds and lichens provide enough light to make it appear as if it were daytime if not for the infinite nothingness beyond the clouds. This is only really notable in pointing out the abyss can support life.
- And, of course, gravity just sort of vanishes past the ledge. It's a slow transition, but only slow enough to get a bit of an arc when throwing something or jumping off the ledge.
There are two major questions:
- What could be the reason for a chasm so large (potentially miles/kilometres across in all directions but up) existing underground?
- Furthermore, chunks of land ranging from tiny flecks of stone to small mountains were left despite this.
- And more importantly, why would it have some of its properties, such as super-viscous air and near absence of gravity?
Answers are preferred to have a scientific basis, but given the high fantasy setting and frank implausibility, magic is also acceptable - especially if it has a good excuse (err, "lore reason") to be so.