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.
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.