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I'm working on a completely flat, underground world, where "world" means "universe" rather than the equivalent of "planet". Here's what I have so far:

  • The world is a flat, apparently infinite (in two dimensions) strip of rocks, metals, dirt, etc., between (seemingly) two bodies of water. There are tunnels, caverns, etc. throughout this strip of solids which are mostly interconnected. This strip is (on the scale of world) quite thin (fewer than 100 km from ocean to ocean).
  • There are distinct "down" and "up" directions which correspond to the direction of the gravity-like force (which I will call gravity) and the opposite of that direction.
  • Gravity is consistently perpendicular to the "plane" of the world because the world actually is flat.
  • The axis of space that corresponds to the "up" and "down" directions is finite and "loops". That is, if someone descended exactly downward through the solids, liquids, and gases of this world, they would eventually return to their original position (hence the two oceans being the same ocean). I suspect this could be described somehow with higher dimensions of space, but I don't have the expertise to attempt that.

The idea is that this world either did not "form naturally" based on physical laws or did so based on laws which are different from those in our universe.

I've added a quick sketch below. This is a cross section where the vertical axis of the image represents the up-down axis in the world and the horizontal axis of the image represents some axis perpendicular to that.

A band of brown with poorly-drawn tunnels between two blue sections labeled as oceans

In thinking about this world, I've realized that I have some issues to overcome:

  • Reconciling gravity with a "vertical space loop" in a way that doesn't result in either total "freefall" or the structural unsoundness of the "underground" part of the world
  • The introduction of heat and light to allow things to live
  • The introduction of water to allow things to live (without the destruction necessarily resulting from holes in the "bottom" of the ocean)
  • Air pressure in a world without a clear end and without the convenient properties of planetary gravity

I have some ideas about gravity (some portion of the world that is constantly accelerating "up" or some portion that is "fixed" in space), heat and light (vertical bars that are very hot and bright), and water (based on previous comments and answers, mostly making the "ocean" much shallower and allowing for infinite rivers from the ocean to itself). However, this question is about the last point above.

In this setting, how can there be sufficient air pressure in the cave systems to allow for people (not necessarily human) to live?

Note: I marked this question #physics because I ideally want answers with explanations based on real-world physics. If the answer is necessarily magic/non-real physics, please explain why.

Big Note: I've totally re-written this question based on comments, which is why there's a lot of ancillary information.

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  • $\begingroup$ If there is perceptible gravity on the vertical axis, what keeps the bulk of the rock and ocean from accelerating in free fall around the finite dimension, thus eliminating the experience of gravity? $\endgroup$ – Logan R. Kearsley Nov 8 at 6:27
  • $\begingroup$ Is there a reason you didn't just make your world a Dyson Ring? It would solve a lot of your issues. The horizontal axis would be so large, it may as well be infinite. $\endgroup$ – Hink Nov 8 at 9:06
  • $\begingroup$ I've just rewritten the question with some more information. Please let me know if this is still unclear in any way. $\endgroup$ – Kinxer Nov 8 at 16:20
  • $\begingroup$ @Hink Mostly it's that I want the setting to be fantastic and non-planetary. I want magic in the setting, but I find that magical (from the reader's perspective) setting elements are more interesting if as much of the setting as possible is consistent with real-world physics. $\endgroup$ – Kinxer Nov 8 at 16:22
  • $\begingroup$ The vertical axis problem leads me to believe your people are 4th dimensional creatures that live on a parabolic time curve of their flat world. $\endgroup$ – Stephan Nov 8 at 20:46
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Air Pressure Follows the Same Rules there as on Earth

Atmospheric pressure is largely the effect of the amount and composition of gasses, and the effect of gravity on the gasses. Due to how fluid pressure works, the use of tunnels is largely irrelevant and can be safely ignored.

Assuming your world is not infinite horizontally, but is rather arbitrarily large (like a Dyson Ring), has an atmospheric composition similar to Earth (including composition of gasses, and gas to volume ratio), has Gravity similar to earth, and is a closed system so air can't escape: air would function like on Earth and would be livable in some areas. That's a lot of assumptions, but you want humans to live there, so it's not going to be too far off and still be habitable.

As for livable areas, the higher you go, the less air pressure. At around 2,500 meters up people altitude sickness can start, at 8,000 meters the air would be unbreathable. This question answers how deep they could go. 2.6 miles is our deepest and is extremely dangerous for humans. I would estimate around 5-10 miles vertically of traversable space, with the majority of people living within a mile of sea level atmospheric pressure, where it would be most comfortable.

For having it livable, it is just getting a spot where air pressure, temperature, food, and water are all available.

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  • $\begingroup$ Thank you for your answer. The altitude notes are particularly useful to me, and I'll think about what it would mean for the world to be arbitrarily large rather than literally infinite. I may end up accepting your answer, but I'm going to give it another day or so to see any other answers. $\endgroup$ – Kinxer Nov 8 at 16:32
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An infinite flat surface would not work from a gravitational perspective as the surface would disintegrate under gravity into lots of small planets or if there was no rotational movement a single ever expanding black hole. So best to use a planet or Dyson ring.

The habitability of the tunnels would depend on a lot of factors: the depth of the ocean, the permeability of the rock, the amount of volcanic activity, geological stability and fault lines for starters. Such a system would only be liveable if you assume a shallow ocean, highly impermeable rocks, very stable geology and not fault lines. Which I suspect is unlikely, but if that were the case people could live there.

If the cave system was large enough air pressure differences between locations would even themselves out over time.

If you were thinking of a deep water ocean like those on Earth then there are extreme problems. The pressure from the water above would be huge and would probably rupture at least some of the tunnels causing catastrophic flooding. All it would take is one serious leak and water at 100-1000 atmospheres pressure would rush into the cave system. All the caves would be explosively flooded in short order whilst the atmosphere in the caves would be compressed into a few small high pressure areas at 100-1000 atmospheres pressure.

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  • $\begingroup$ Thank you for commenting on the ocean issues in particular. I'll be integrating those in to the world (no doubt the ocean will need to be much shallower). I'm intentionally avoiding planetary geophysics by essentially handwaving (the origin of the world doesn't result in plates, for example), but I appreciate the comments on those as well. $\endgroup$ – Kinxer Nov 8 at 16:29
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    $\begingroup$ the tunnels would also collapse under the weight of the rock around them. tunnels have to be very shallow to not shrink over time. solids are not as solid as people think. $\endgroup$ – John Nov 8 at 20:21

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