What would a literal "under-world" look like?

[Greywander]

You've delved deep into the earth, deeper than any before you, past twisting caves and voluminous caverns, beyond the boiling lakes of fire, and yet deeper still. The air is cooler now, and the unexpected scent of fresh air catches your nostrils. As you break through the last layer of rock and dirt, you're greeted by an open space. No, the open space. The vast expanse of the sky stretches out below in a dizzying drop into nothing. The bright orb of the sun briefly blinds you, indicating that it may be night on the surface above. After all this time, you've finally found what you could only have speculated on before: the world's underside!

I was thinking about this and initially thought that maybe the underside would just duplicate the surface, but upside down, but quickly realized that that wouldn't work for a number of reasons. Now, there are a couple different types of "underworlds". Is it a hollow-Earth or the underside of a flat world? If the former, is it a giant cavern or is there a sun-analog to give light? Is gravity reversed or does everything hang down from the earth above? For now, I'll just assume a flat world without reversed gravity (as reversing gravity would pretty much make the underside the same as the surface).

The first thing that occurred to me is that most animals probably either fly or are really good at climbing and jumping.

However, if we want to have life in the first place, rather than a barren wasteland, what we really need is water. Because the ground is upside down, there can't be any lakes or rivers. You can have streams that flow out of the underground, but it will just fall into the sky. I can imagine having a thriving ecosystem gathered around such points, likely with plants designed to catch as much of the falling water as possible, but these ecosystems would be fairly small. We also can't have rain, unless we contrive a mechanism by which the rain falls up instead of down (strong wind, perhaps?). This also begs the question of what weather would look like in such a place; I imagine rain falls down but lightning goes up.

What I suspect to be the mostly plausible method for watering this underworld is dew. Basically, mist/clouds form close to the "ground" and condense into water. I can see having plants that try to maximize surface area so as to condense the most water, or they just soak up the water and swell up really big (like succulents). Trees might have branches that bend back toward the underside, with leaves shaped like bowls to catch and hold as much water as possible. Perhaps vine-like plants hang down into the sky to try and capture any moisture from the air. I wonder if the clouds would be a lot closer to the underside than they are to the surface; if so, then it might be possible for such a vine to reach the cloud layer and siphon water out of the clouds.

Floating islands, if they exist, could also change things a lot. They would give a place for things like lakes and other features that could only exist on the surface to form. These floating islands might catch the water flowing out of underground streams, and the vine-things might hang down into the lakes and suck the water back up to the underside. Animals could also drink from these lakes, and might even make homes on the floating islands.

That's about as far as I've thought this out. Lots of hanging plants and stuff, streams flowing out of the underground that are hotspots for life, and other weird stuff that is designed to make life on the underside plausible, if fantastic. Figuring out where the water comes from seems like the biggest first step, which will then lead to emergent world building where plants and animals will gather near the water and various systems and subsystems will build themselves as a natural consequence to these things.

[Tvtyrant]

Hmm, maybe the spin of the planet pins everything except the core itself to the inner "surface" of the planet. The core acts like a sun that is permanently stuck at noon, rotating in place as a hot dull lump of iron. Water vapor and clouds are turned into steam and blown back to the "surface" as hot vapor so it's very lush and tropical without a lot of standing water. Like a green house space colony.

[sktarq]

Well a couple of things that would seem to be an issue....firstly underground water heading toward the surface...so plants basically can't use roots to tap water...also speaking of roots...roots break stone down to soil and you can't have soil...it'll just fall away. Between these you are going to have serious issues with plants...you may be able to pull off lichen but not much more...
Also you're going to have a problem with air pressure....figuring out where the air and water "go to" becomes an issue because otherwise the air just fades away and nothing can breathe.

[Grek]

I'm currently running a game set in a world like this, and my solution to this problem was to have the universe be of finite volume, with there being a large but finite body of water beneath the world. Some parts of the underworld see the roof dipping down into the undersea, while other parts are dominated by tremendous leafy plants that span the distance between Roof and Sea. The light shines up from the rippling waves, released at a constant rate by the cosmic singularity where the universe narrows down to a single point at the bottom of the world-cone.

[brian 333]

Disc-world shape, floating in the void is orbited by the sun and moon(s). The sun is smaller and nearer, so its orbit is daily. It can eclipse the moon, but because you can't see the moon when the sun is up, only astronomers on the edges of the world, (which appear as a massive mountain range to those who go there,) can observe a solar eclipse of the moon, though lunar eclipses are more frequent.

The moon is also a disc, and its top and bottom align with that of the world, so that it waxes and wanes as it travels across the night sky.

The atmosphere is a globe, thinner at the world's rim. When water falls from the world it breaks down to a mist which is heated by the close passage of the sun, forcing it to flow as steam back to the top where it falls as rain. The orbit of the sun is nearer in the summer and more distant in the winter causing the weather to be more regular in summer and mose violent in winter.

Nightly dews and fogs are the common source of water on the underside as steam which did not make it around the edge is drawn into the cooling center of the disc. Plants anchor to the rocks and dangle, capturing moisture in various ways.

Underside is the domain of flying things, but terrestrial creatures, especially brachiators, thrive in the dense jungles which form around the seeps and waterfalls. These 3D mazes are separated by open expanses not unlike moonscapes. The craters are where chunks of the world have broken off.

When a chunk breaks it forms an island with a mini-ecosystem not unlike that of the world. It slowly falls for decades or centuries, its top growing ever more lush as it is ground into dust by the elements, eventually to be caught in the steam clouds and deposited on the surface by the rain.

[Bohandas]

What if the planet is just a big dirt clod clogging up a wormhole, and so the "underworld" is just another outer surface somewhere else in space

Hmm, maybe the spin of the planet pins everything except the core itself to the inner "surface" of the planet. The core acts like a sun that is permanently stuck at noon, rotating in place as a hot dull lump of iron. Water vapor and clouds are turned into steam and blown back to the "surface" as hot vapor so it's very lush and tropical without a lot of standing water. Like a green house space colony.

I like this idea too

[PoeticallyPsyco]

Hmm, maybe the spin of the planet pins everything except the core itself to the inner "surface" of the planet. The core acts like a sun that is permanently stuck at noon, rotating in place as a hot dull lump of iron. Water vapor and clouds are turned into steam and blown back to the "surface" as hot vapor so it's very lush and tropical without a lot of standing water. Like a green house space colony.

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.

[Tvtyrant]

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.

[PoeticallyPsyco]

Okay, having now rewatched that video I linked and done a little bit of comparing with Schlock Mercenary's Eina-Afa (link is to wiki page, which itself has a handy reference to links in the comic about people discovering/realizing something about the superstructure), which is a 'habitat' on a scale so huge that it has a lot of the same features as a hollow planet, there's a few tricks you can use to mitigate some of the problems from my previous post.

The Coriolis Effect: not much you can do to get rid of it entirely; anything moving through the air without holding on to the ceiling is going to have its trajectory changed by about 30% near the equator. You can reduce that by making the planet larger (which will make it a much longer journey to reach zones with significantly different physics, which could be a good thing or a bad thing) and by slowing its rotation (reducing 'gravity' but also reducing the Coriolis Effect). OTOH, if the core is also producing actual gravity pulling inwards, the spin and therefore the Coriolis Effect will have to be much stronger to compensate at the equator (this will also make it so near the poles the ground is a ceiling, near the equator it's a floor, and in the middle is a weird slope).

The weather: the combination of a hot core, a cool exterior, and extreme Coriolis Effect is going to result in some bonkers weather. You'll get a lot of hurricanes and tornadoes, but close to the equator they'll actually be parallel to the ground rather than perpendicular to it (if a regular hurricane is shaped like a flying saucer hovering over the ground, these would be more like a wheel rolling on the ground). You can reduce their severity by including wind-blocks, which in this context, would probably be enormous pillars stretching from the core to the ceiling, and maybe interconnecting as well (this also handily explains how the planet has any kind of structural integrity). Each of these pillars would be big enough to need its own wind-blocks, most likely taking the form of giant walls separating the surface of the pillar into sections.

[brian 333]

I once 'designed', (plagerized,) a generation starship which was a massive cylinder that rotated for gravity. Its center was a massive cavern 3 1/2 miles in diameter and 10 miles long with a half-mile thick hull designed as a labyrinth of compartments for every purpose except farming, which was done in the center chamber. Along the axis was a tube through which a fusion reaction flowed, fed by Bussard ramscoops and supplemented by internal fuel tanks. The tube glowed white-hot when the available fuel was abundant and faded to red when the engine was running on onboard fuel. The engine provided electricity, light, and heat, but very little thrust. The 30 ly journey was planned to be completed in 900 years.

The floor of the cylinder was at .75g, but it reduced as one climbed the mountainous ends. Landing on the spinning cylinder was very tricky, (my players crashed into the small craft launch bay,) but launching was easy.

One could imagine this idea scaled up to world-size, with a central core containing the fusion plant which is suspended in the zero g zone on massive pylons which are invisible from the equatorial zone. Thus the mass of the 'sun' would not be great enough to negate even a tiny fraction of the centripedal pseudo-gravity. Even better: the sun can be regulated to be bright and hot in the day and to be turned off at night, with twilight as it heats up or cools down. By intensifying or dimming and longer or shorter on times, seasonal variety can be simulated, though it will be the same season everywhere in the world.

This can be made a magical artifact with a little thought. The sun is maintained by an efritti clan and its fire elemental servants who constantly wage war against the earth elementals along the length of the pylons, or something.

[Trask]

Go watch conspiracy theory videos on youtube about "The Hollow Earth" and "Agartha". Great source of ideas.

[Greywander]

Hollow Earth is one of the possible types of underworlds I proposed in the OP. The other one, which I haven't seen as many people talking about, is the flat Earth with an underside. In this scenario, there is a normal day/night cycle, where it's day on the underside when it's night on the surface above, and vice versa (because the sun is literally passing under the planet).

There is the matter of what happens to things that fall off the underside. If the planet is a flat disk, and gravity still points "down" into the void below the underside, then gravity must work differently than for a normal planet. Gravity might be more like a magnetic field, with "poles" that cause it to be directed. The "up pole" is on the surface, while the "down pole" is on the underside. This means that as you move away from the surface/underside, especially if you're far from the center of the disk, then gravity will start pushing sideways. If something falls off the underside, the gravity lines will eventually curve all the way around the disk, causing whatever fell off the underside to take a very long path back to the surface. This could also explain floating islands; it's essentially the same idea as magnetic levitation. This might mean that floating islands are only stable in certain configurations, and that outside those configurations they will drift, and may flip over and crash into the surface or underside.

I imagine this change in gravity would also affect things on the surface. Mountains or tall buildings might need to tilt to one side in order to stay upright, for example. In fact, it should be possible (if you have strong enough materials) to build a tower that goes from the surface to the underside, curving around the disk by following the gravity lines. Also, if things that fall off the underside end up back on the surface, then we could use the idea of having a layer of magma between the surface and the underside that's responsible for tectonic activity. Perhaps some parts of the surface are "sinking" into the magma ocean, and in turn being used as construction material for the underside, which then gradually erodes away and is deposited again on the surface.

I wonder if this "magnetic" gravity would have an effect similar to the Coriolis effect? Given that gravity will change direction as you move away from the surface or underside, it might not be that different from how it was presented in that video about gravity on Halo. Assuming that gravity lines emanate from the entire disk, and not just from the poles, then gravity would be mostly normal close to the surface/underside, so projectiles wouldn't experience much of a problem as long as you're aiming for someone on the ground. If you shoot into the sky (either above or below), then the bullets/arrows/whatever might take some weird paths.

[Yakk]

Down is a real direction.

Things made of elemental Earth pull things down "above" them. Heavier things are pulled harder.

The world is a big rock, floating in air. There is an edge, and a bottom, and a top. Things on the top fall towards the elemental Earth of the rock. Inside, similarly, you are pulled down.

When you reach the bottom, or go past the edge, you stop falling. There is no (nearby) elemental Earth to pull you down anymore. So you can build structures that hang off the bottom of the world (or the side).

Water shields pulling down. It shields it best when you are embedded in water, but to a lesser extent when above it. Airships, ships that fly between worlds, can take off from the edge of the world (or the bottom); on the top, they can descend above large lakes or rivers, using the reduced pull to be able to get lower without crashing. Once they land, however, the pull of Earth above the water is enough to prevent take off.

So airship trade flies above the world, then down into lakes and rivers, then down the lakes and rivers to the edge, then over the edge.

The water that flows over the edge of the world forms new storm systems when mixed with elemental air, and those storm systems in turn deposit water when they are above your rock of the world.

[PoeticallyPsyco]

Down is a real direction.

Things made of elemental Earth pull things down "above" them. Heavier things are pulled harder.

The world is a big rock, floating in air. There is an edge, and a bottom, and a top. Things on the top fall towards the elemental Earth of the rock. Inside, similarly, you are pulled down.

When you reach the bottom, or go past the edge, you stop falling. There is no (nearby) elemental Earth to pull you down anymore. So you can build structures that hang off the bottom of the world (or the side).

Water shields pulling down. It shields it best when you are embedded in water, but to a lesser extent when above it. Airships, ships that fly between worlds, can take off from the edge of the world (or the bottom); on the top, they can descend above large lakes or rivers, using the reduced pull to be able to get lower without crashing. Once they land, however, the pull of Earth above the water is enough to prevent take off.

So airship trade flies above the world, then down into lakes and rivers, then down the lakes and rivers to the edge, then over the edge.

The water that flows over the edge of the world forms new storm systems when mixed with elemental air, and those storm systems in turn deposit water when they are above your rock of the world.

That... actually has some really interesting implications for worldbuilding. The lack of gravity below and around the floating island will have all sorts of effects on weather, but the biggest and most notable is that air above the island will constantly be flowing down until it reaches the ground, then being pushed to the outside of the island by the air above it, at which point it will flow upwards to replace the air being pulled down, creating a constant cycle of wind on the island and enormous updrafts surrounding it. Conveniently, this should also ensure that most water (and anything else for that matter) that falls off of the floating island-world will eventually end up above it and be pulled back down, ensuring the island doesn't become a desert once all the water flows off.

Another effect is that the island will be constantly pushed downwards, even if Earth doesn't pull on other Earth, by the air and objects on top of it, accelerating until it reaches terminal velocity (which won't be nearly as fast as 120mph, but would still be pretty dang fast without something more than air resistance to push upwards against it). Since the rate of fall will be pretty constant, people on the island won't notice (they're all moving at exactly the same rate and the island isn't accelerating beyond unnoticeably small changes caused by wind), but the stationary air outside the island will appear to be moving upwards at quite a clip even once you get past the updrafts caused by the air-gravity cycle (and those updrafts will be much faster relative to the island as well). Step/fly off the island and it'll look like the winds are shooting you upwards to someone on the island. Where this gets really interesting is that if there's more than one floating island/chunks of rock, they won't all fall at the same rate; the wider the island, the slower it will fall, which will create an interesting dynamic if you have multiple such inhabited island-worlds nearby each other. Clever engineers could even build off of an island by taking this into account, using just the right amount of elemental earth relative to the weight on top of it (and relative to the updrafts pushing from below) to create mini-islands that fell at the same rate as the main one, essentially floating alongside it. Last but not least, the fact that the island is moving downward means that the underworld will be buffeted by wind from below, though not actually as much as the overworld (as long as Earth-gravity doesn't affect other Earth, see below), which will interact with the topography to create winds in all directions, though generally moving towards the edges of the island.

Okay, above I mentioned Earth not pulling down on other Earth a couple of times. I think that that's a good rule to add to the setting, (perhaps with the addendum that this only applies as long as they are connected, so separate bodies of Earth will still pull each other and individual rocks will fall if dropped etc., but this change won't actually make a huge difference from a worldbuilding perspective). The reasons I think it's a good idea are twofold. First, without either this rule or some other force to push upwards on the floating island(s), the terminal velocity of the islands will be much higher, since there's now much more weight pulling down the thicker the island is, but the surface area of the bottom (and thus the coefficient of air resistance) stays the same. That makes cool ideas like the airships you discussed much less practical, and travel between islands difficult to the point of probably just requiring teleportation (although... actually, I guess you could just take some rock with you so you're falling too and the islands don't just fall away from you faster than you could ever catch up. So maybe not as impractical as I thought). Second, it allows for some other cool worldbuilding elements like fantastical stone architecture unbounded by gravity, smaller floating islands/mountains above/below/around the main island-worlds, and the like.

And lastly, you might want to make it a 'looped' universe like the Everfall setting, so that if you fall far enough you end up back where you started. Makes it possible to do things like re-encounter a civilization left behind hundreds of years ago because your islands were falling at different speeds, or find an ancient relic from a previous age that just fell out of the sky one day (and is actually part of the world's history rather than just coming from an unknowable and unreachable place high above, though that would still be possible).

So yeah, all in all that makes for a cool and surprisingly consistent setting. I'd definitely read a story set there.

[Yakk]

Oh, I wasn't going to have things which fall towards Earth to end up pushing it. Equal opposite probably; the stuff falling towards the Earth ends up pulling the Earth up equally, just in a distributed manner, that is cancelled by the impact that happens anyhow.

The range of the pull is intended to be limited. An "island" would produce a gravity field that was maybe 1000' up, and it might weaken as you went higher, and definitely weaker above rivers and water, because I just love the idea of trade ships flying over the land and landing on lakes inland, or in an emergency trying to land in a large river.

As I'm story-first, and not physics first, the field would tend to grow strength as the amount of rocks is added, then plateau in strength, but continue to grow in size as the island/continent gets larger. This is because having 1 gravity is useful. ;)

That means a small island, or even an airship, could have 1 gravity on the deck/surface, just over a limited height.

[PoeticallyPsyco]

Oh, I wasn't going to have things which fall towards Earth to end up pushing it. Equal opposite probably; the stuff falling towards the Earth ends up pulling the Earth up equally, just in a distributed manner, that is cancelled by the impact that happens anyhow.

The range of the pull is intended to be limited. An "island" would produce a gravity field that was maybe 1000' up, and it might weaken as you went higher, and definitely weaker above rivers and water, because I just love the idea of trade ships flying over the land and landing on lakes inland, or in an emergency trying to land in a large river.

As I'm story-first, and not physics first, the field would tend to grow strength as the amount of rocks is added, then plateau in strength, but continue to grow in size as the island/continent gets larger. This is because having 1 gravity is useful. ;)

That means a small island, or even an airship, could have 1 gravity on the deck/surface, just over a limited height.

Okay fair enough. Capping at 1g is also probably a very good idea.

You would still end up drifting downwards, due to the wind cycle created by localized gravity (all the wind above the islands will be more or less pointed down, pushing the island ever so slightly). Would be pretty slow, though. EDIT: ...or maybe not. Actually, I think they'd end up drifting upwards ever so slightly, pulled upwards by the equal and opposite force of gravity. Either way, it's probably negligible, and trying to model it in my head is making said head hurt.

[Cluedrew]

Speaking on the disk-world format: I think the biggest issue I have with the "underside of the world" settings is: Where do things fall? Or rather how do things not fall away forever?

Looping is a solution: Falling stars are actually just rocks dropped from below and there is actually always pits of dust falling from the bottom to the top. Another is that gravity does reverse but far below the main body, so there is a low gravity area you can go to find anything that has falling. Actually any sort of "net" would do. You would probably want it to be mostly inhospitable - unless you want another society down there. The other thing you could do is have objects reverse their "local gravity"

Also I recommend batfolk as the primary inhabitants of the underworld. They just seem to be the perfect ones to address the upside-down nature of the world, the reversal of night-and-day and they can fly which is useful around there.

[Greywander]

Speaking on the disk-world format: I think the biggest issue I have with the "underside of the world" settings is: Where do things fall? Or rather how do things not fall away forever?

This is definitely a tricky one, particularly if the oceans are flowing over the edge. Where does the water go, and how does the world not run out of water? I proposed a possible solution to this in my previous post, where gravity acts more like magnetism. The disk is like a round, flat magnet, emitting field lines from one side that curve around to the other side, but these aren't lines of a magnetic field, but rather a gravity field. So anything that falls off the bottom will go through a wide curve and eventually fall back on the top. This has some interesting consequences, for example if you build straight up then gravity won't point straight down, and the effect becomes more pronounced the further up you build or the farther you are from the center of the disk. So tall buildings will probably curve away from the disk's center slightly.

[Geordnet]

From what I understand, the gist of the idea is "keep digging down, until you break through". This precludes any sort of centrifugal force or reversal of gravity, since that changes the direction of "down" back towards the original surface. That leaves three options for gravity:

1. Everything has to cling to the under-surface, or fall into... an endless void, or something else?
2. Everything floats in zero (net) gravity "beneath" the under-surface (as should be the case inside a hollow sphere irl).
3. Gravity stops behaving consistently, and just goes in whichever direction if feels like.

All are pretty interesting scenarios, but of course need a significant dose of magic to make work. I'd say start by looking at what hard sci-fi authors speculate for alien life that lives in a gas giant/ring system, or hanging underneath an ice sheet, and then add exotic magic to that.

Maybe mix in references to myths where the world is literally built upon/from the body of some primordial being the gods fought and (hopefully) killed...

Oh, and consider that the under-world doesn't have to be filled with air (or vacuum). You can use a different fluid, such as water, magma, crude oil, unbreathable gas, etc.

Hm... Has anyone seen the movie "The Core"? Cross that with "Journey to the Center of the Earth", and that sounds like a memorable campaign.

[Melayl]

2E had the "Hollow World" setting, which is pretty much what you're describing. I don't know difference you can find it anywhere anymore, though...