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I would like a world with a big LOT of subterranean space to explore, colonize, and trap ancient evil beings.

Now, while the questions about food and ventilation are well adressed on Worldbuilding, I would like to ask the question geologically speaking.

From what I understand, caves on Earth can be :

  • Limestone cave
  • Lava Cave
  • Artificial caves (quarry...)

The first one exists with system up to 200 km large. The second seems more anecdotic. The tird one doesn't concern us.

What I'm asking for :

What types of cave could we encounter in a World like this, what conditions would create them, and what geologic impossibilities could I encounter ?

What I thought of :

More limestone, More ancient world, allowing more time for nature to carve these caves, More Volcanic activity, More Rain

How can a world with somewhat similar surface to the Earth's one develop these very large underground formations ?

Edited : I did send a sketch at first, sorry.

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    $\begingroup$ Lava tunnels are far from being anecdotes. Lava tubes can be up to 14–15 metres (46–49 ft) wide, though are often narrower, and run anywhere from 1–15 metres (3 ft 3 in–49 ft 3 in) below the surface. Lava tubes can also be extremely long; one tube from the Mauna Loa 1859 flow enters the ocean about 50 kilometers (31 mi) from its eruption point, and the Cueva del Viento–Sobrado system on Teide, Tenerife island, is over 18 kilometers (11 mi) long, due to extensive braided maze areas at the upper zones of the system. $\endgroup$ – L.Dutch Sep 13 '17 at 8:15
  • $\begingroup$ Limestone, on that scale I believe, implies lots of oceans. Make sure that makes sense scale-wise. I would probably suggest something with tectonics. to create rifts. Then more tectonic forces to slam on top of the rifts. Better yet, have your tunnels arise from a variety of sources, it's more realistic if a variety of forces contributed. I like the idea of them being quarried. Star Trek did this with a silicon-based horta. $\endgroup$ – DPT Sep 13 '17 at 15:13
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Could this be a future-type setting with an earth ravished of resources, the huge gaping caves remnants of heavy mining operations (of different kinds)?

Otherwise, an event within our solar system has caused large-scale earthquakes, weakening the structure of the crust but creating a 3d lattice of mega-caves, some even filled with sea water. You wouldn't have to explain the event, you could call it an "unknown event".

Just some ideas for you to ponder.

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If there evolved a very efficient coal eating bakteria that consumed all the coal deposits and turned them back into CO2 leaving huge empty spaces. This should probably be a quite recent event say in the last 10 million years so that it was not all destroyed by tectonic activity and erosion. Also bear in mind that only the caves that are above the local ground water level will be filled with air, this probably translates to mountanous areas and other highlands.

Edit: Another cool posibility is that the process is still going on and that the bacteria has evolved a symbiotic relationship with some insects that farm them. The caves could the be explained in story as all this was excavated over tousands of years by the rock eater wasps, they are still going at it by a meter a year in the lower reaches. The idea being that the wasps suplies the bacteria with mutrients that are hard to come by in the coal seams like phosphourus.

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  • $\begingroup$ Arg, you got a nice point with the air thing. Do you think that under sea level caves are totally impossible, even in a continental area ? $\endgroup$ – GlorfSf Sep 13 '17 at 9:01
  • $\begingroup$ Hard to tell, I no geologist $\endgroup$ – lijat Sep 13 '17 at 9:06
  • $\begingroup$ Insufficient nitrogen. Your balance is out of whack. Also need to account for sulfur and oxygen. If the bacteria are anaerobic, then you run into some thermodynamic problems, but these are less of a problem. $\endgroup$ – DPT Sep 13 '17 at 15:07
  • $\begingroup$ @DPT Nitrogen could be fixated from the air could it not? As well as oxygen. Sulphur would probably need a solution similar to phosphorus. $\endgroup$ – lijat Sep 13 '17 at 15:54
  • $\begingroup$ @lijat If the bacteria are eating coal (tons of carbon) and leaving caverns, they need access to the atmosphere to have enough N to fix. So that access needs to be worked into the design. If there are areas underwater as suggested in the answer, there will be N limitation (no air access) and likely P and S limitation as well. I think the balance is pretty far out of whack, this does not mean that bacteria don't eat coal, but that there are nutritional constraints upon it, which is probably why we have 250 my old coal beds that aren't riddled with happy bacteria tunnels.... $\endgroup$ – DPT Sep 14 '17 at 15:07
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Massive Aquifers left dry after some event which reduces the water level of the planet (by a hundred meters or so.) Think a heavily ocean-covered world, 80% or more surface water, and then a rapid warming or maybe a glancing blow from a large-ish meteor sublimates or evaporates much of that surface water.

The "hard to believe" part here, though, is that massive caverns tend to collapse once whatever was supporting them is gone, and in the case of aquifers, this is often water pressure providing said support.

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Continental plates are moving, raising and sinking, sliding over each other. They consist of layers of different minerals.

Image a plate consisting of three main layers: one directly swimming on the magma in the deep, above that a layer of fragile minerals, and on top a quite stable, somehow slightly elastic layer.

This continental plate partially slides on another and is raising. The raised part not already lying an the other plate does not lie directly on magma anymore. The continental plate splits horizontally along the fragile middle layer. The inferior plate falls back onto the magma. The stable, slightly elastic layer is hold up because it lies partially on another continental plate.

Between superior and inferior layer is a gigantic cave with the debris of the middle layer. It will be partially filled with magma (and sea water?) until all calms down.

The superior layer must be stable and slightly elastic, otherwise it would just break down. I am curious how this could be achieved in a geological way.

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