## Re: What would a literal "under-world" look like?

Originally Posted by PoeticallyPsyco
I like the idea of a hollow planet with a glowing core substituting for the Sun. However, centrifugal force as a replacement for gravity has the potential to cause all sorts of weirdness... which is not necessarily a bad thing, but is something to be aware of.

For starters, using centrifugal force to push people onto the 'ceiling' won't give the aesthetic of an "underworld"; in the places where that works it'll just be a regular world with the quirks below.

Big-scale stuff. Centrifugal force wouldn't be constant across the surface. It would work basically okay near the equator, but at the poles there wouldn't be any spin at all (that's why you can balance a spinning basketball on your finger), and thus no force. Also, in between, the force wouldn't be pointed at the ground, it would be pointed straight out from the axis of rotation. So at a latitude of 45 degrees, 'gravity' would be at a 45 degree angle to the 'ground'.

However, even at the equator, you're going to get some serious weirdness due to the Coriolis effect. On Earth, the planet's spin is slow enough that we don't notice it, and the Coriolis effect mostly only impacts the weather and people trying to play catch on merry-go-rounds. However, with fast enough spin to replicate gravity, you'd notice it, as you and the ground are constantly curving through space, while anything airborne will be moving in a straight line. Arrows, catapults, heck, even flying birds would all move extremely weirdly to someone from Earth. Here's a video on how that would work on the Halo rings (if they didn't use artificial gravity), which are close enough in scale to Earth for our purposes.

The physics complaints can be ignored if your players/audience doesn't care about that, but you'd be losing out on most of the worldbuilding opportunity by making it "basically like the surface of Earth, but instead of a horizon it curves upward".

So. With a 'ceiling' but no 'floor', preventing falling is going to be the name of the game for all life... and geography, for that matter. That means most creatures will have at least limited flight.

Note that creatures using wings for flight can't fly forever; they need to be able to land and rest at some point. Most bird-, bat-, and insect-like fliers will therefore be limited to the 'ceiling' biome, floating islands (if those exist), and perhaps symbiotic/parasitic relationships with creatures/plants that can fly indefinitely.

Indefinite flight could be powered by gasses (like blimps), maybe updrafts, and probably at least a few that are straight up magic. Creatures using giant gas sacks will likely be slow and unmaneuverable (unless they can switch to winged flight, deflating their sacks temporarily to become nimble at the cost of energy). Updrafts aren't necessarily consistent; anything relying on those for altitude needs to either be able to fly to a new updraft or endure 'sinking' for a potentially very long time until it stumbles into one.

'Sinking' probably does need to be a bad thing for some reason or other, though, or else there would be no reason for anything to live at the ceiling (where the story presumably takes place) instead of down closer to the energy source that is the core (like how on Earth, very little lives in the depths of the ocean compared to the surface). The easiest explanation is probably that the core is extremely hot; so while getting closer does increase the available energy, it also makes the environment harsher, until you get close enough that plants can't survive at all (and then there's no direct food source for the animals, though some would still live in this plantless zone, feeding on scraps drifting down from above).

Other ideas for preventing falling are octopus-like suckers, gecko-like cling pads, magnetism (probably my favorite of the bunch), roots extending far up into the ceiling, and just being able to climb on plants/animals/geography that can avoid falling.

One final worldbuilding problem (that I can think up at the moment at least): weather and erosion. This space is presumably huge, mind-bogglingly so. It's going to have its own weather, and just like how on the surface winds get faster the higher up you go, once you leave the ceiling biome there's nothing to slow down the hurricanes. OTOH, winds on the surface are largely caused by how land and sea heat up at different rates; without oceans on the ceiling, perhaps there wouldn't be as much total air movement in the underworld? I'd buy that. However, even a slight amount of weather will cause erosion. Not necessarily a problem, but if you do want there to be variety in the geography and terrain, you need to come up with a reason for it to keep being created even as it gets eroded away over time. Perhaps there's a layer of magma between the surface and the underworld, so there can still be tectonic activity, or perhaps pieces crumble off of the ceiling and fall into the core (or become floating islands), leaving behind landscapes (ceiling-scapes?) more interesting than a mostly smooth surface.
I like those ideas as well! I think you could do a cool combo one: The centripetal zone where things are held against the ground, and much larger hemispheres that they aren't. I would base it on the water to core circulation: Hot steam blasts off in big weather events and turns into vapor that is blown against the walls of the world, and then drips back to the core. The life in the areas outside the equator are built like coral reefs, with the life burrowing ever further into the rock and using calcium and other minerals from the rock to cement themselves down. Flying life forms occupy the reefs, hiding when steam blasts come out and hunting each other/eating the reefs themselves.

This solves the erosion to an extent; it won't be smooth because the environment is made of life form shells. Weird twisty pillars and barnacle like designs that are immune to steam and resist erosion. Tentacled clinging organisms climb them, and humans have to rock climb hanging inverted if they want to traverse it.