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The Old Ones build continent-sized bunker complexes to protect themselves from the enemy, which followed through the cosmic ocean. The Hunter succeeded easily despite their efforts and their corpses were entombed in the deepest vaults by their corrupted servants, so their masters may continue to dream in death. The vaults were later settled and expanded by the Dwarfs, yet their empires were destroyed as well. Now only the stupid, greedy and desperate enter the Underdark.

The bunkers are all roughly funnel-shaped cave systems, with radii between 500 and 1000 kilometers. Their ceilings have been waterproofed, yet they have become leaky over the last seven million years. While the magical superstructure is quite strong, continental drift and earthquakes have taken their toll. The water which flows together at the bottom of every bunker, where it is dispersed back to the surface via teleportation. The Underdark only begins at a depth of one to three kilometers. It is separated from the upper-world. Only few, mostly dwarven access shafts lead down. Underdark dwellers are adapted to the atmosphere down there. Upperworlders can handle it as well, but need acclimatization and can't handle it in the long run.

The Underdark has forests of gigantic, phosphorescent mushrooms, whose light is so weak that surface dwellers must spend at least a day in total darkness to see it. Dwellers of the depths can use it almost as well as daylight. Normal mushrooms are quite diverse down there. The Dwarfs use many species to get food, fibers, leather, and wood. Many species in the Underdark are phosphorescent, providing at least some light in habitable regions. Where the old Dwarfs installed sunstones, woods of ferns dominate. The significantly higher concentration of carbon dioxide in the local air helps them a great deal. The seas and lakes have matts of chemosynthetic, endemic species. They use methane, hydrogen sulfite and ammonia, which are significantly more common in the Underdark. Nutrients are provided by the water, which flows into the underground. All these plants provide food for the horrific beasts of the world below and the Tiefling, Mindflayer and Dwarf-civilisations down there.

While such undergrounds aren't uncommon in fantasy, I want to ground this thing in reality. I think that the Movile Cave makes a good blueprint for such an ecosystem.

The air in the cave is very different from the outer atmosphere. The level of oxygen is only a third to half of the concentration found in open air (7–10% O2 in the cave atmosphere, compared to 21% O2 in air), and about one hundred times more carbon dioxide (2–3.5% CO2 in the cave atmosphere, versus 0.03% CO2 in air). It also contains 1–2% methane (CH4) and both the air and waters of the cave contain high concentrations of hydrogen sulfide (H2S) and ammonia (NH3). The cave is known to contain 48 species, among them leeches, spiders and a water scorpion. Of these, 33 are endemic. The food chain is based on chemosynthesis in the form of methane- and sulfur-oxidizing bacteria, which in turn release nutrients for fungi and other bacteria. This forms microbial mats on the cave walls and the surface of lakes and ponds which are grazed on by some of the animals. The grazers are then preyed on by predatory species. Nepa anophthalma is the only known cave-adapted water scorpion in the world. While animals have lived in the cave for 5.5 million years, not all of them arrived simultaneously. The most recent animal recorded is the cave's only species of snail, which has inhabited the cave for slightly more than 2 million years.

Is the Underdarks ecosystem plausible, given the existence of the Movile Caves one? The Underdark would probably have a bit less CO2 and more oxygen, due to the existence of the fern forests and slightly better ventilation. Would decomposition be enough to explain the ammonia, methane and hydrogen sulfite or are other sources required? Why is phosphorescence so widespread, dspite it being a waste of energy at first glance?

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    $\begingroup$ Well, for starters, it's important that the characteristics of the Movile Cave are dependent on it being completely sealed off from the rest of the atmosphere, and the creatures that live there would probably not do well in normal surface conditions. Certainly dwarves adapted for surface O2 and CO2 levels would NOT be able to breathe at atmosphere like that. A dwarf mine that broke into a place like this likely would treat it as an environmental hazmat situation and seal the shaft off. $\endgroup$ – Morris The Cat Jan 15 at 17:20
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    $\begingroup$ Well, origin aside, if dwarves have been making holes that connect the underdark to the outside world, unless they built airlocks, that's going to cause mixing and, over time, mean that the atmosphere inside your underdark and the surface will equalize. Having a completely different atmosphere in the underdark is incompatible with having lots of open passages to the surface. $\endgroup$ – Morris The Cat Jan 15 at 18:00
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    $\begingroup$ Another factor I don't see accounted for here is temperature. When you start getting that deep into the crust, the temperature goes up dramatically. There's a mine in South Africa that's just barely as deep as what you're describing, and temperatures at the bottom of the shaft are upwards of 130-140F. The atmospheric pressure is also much, much higher than surface levels. en.wikipedia.org/wiki/TauTona_Mine $\endgroup$ – Morris The Cat Jan 15 at 18:04
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    $\begingroup$ "how big their shafts are" - I fail to see how that's related to the issue at hand... I'll see myself out. Hm, the pressure could be approximated via the scale hight of the atmospere. The World was already supposed to have lower gravity than Earth. Lowering the pressure generally and keeping an eye out for toxic concentrations sould help. If the crust is thicker, because the planet is colder (smaller, older, less radioactive elements) and the Old Ones heatproofed the bunkers I'm probably fine. Additionally this makes the lower levels more dangerous and creates boiling seas. The thats great! $\endgroup$ – TheDyingOfLight Jan 15 at 19:05
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    $\begingroup$ honestly this sounds like an amazing scene/place for a hollow knight type game or similar roguelite/metroidvania. ok i'll leave now. $\endgroup$ – michael griffin Jan 15 at 20:11
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Energy

All living things need energy to survive, though they get it different ways. On the surface this is easy - just absorb sunlight. The Movile cave has chemosynthesising bacteria, which is a bit more limited in energy utput and by the face that is gradually eats through the rock. You can see this limited energy availability in that the most complex animals are insects and scorpions. To make bigger, humanoid animals with their higher energy requirements available you need more energy, which you can get two different ways.

One, bigger caves - you already have this covered to some extent. If the production of food is less efficient, you'll need more food-producing area per person - the caves will have fewer people living in them that in a similarly sized area on the surface.

Two, going vegan. A carnivoire eats animals and a herbivoire eats plants, but each step in that process loses a lot of energy (about 90%) meaning that the plants need a hundred times more energy input than the carnivoires do. Every step in the food chain you can cut out is therefore a great gain energywise. This means to be able to grow their society to full potential, our cavedwellers would probably be harvesting and eating the microbial mats directly. Yum!

You menation nutrients coming down with the water. If you get energy from this as well, like algae in the water, that would be an extra energy source and in this environment every bit helps.

Energy hogs

Forests of mushorooms and ferns can use up a bunch of availabe energy and area, limiting what is available to animal species. You can see this on the surface too - a forest usually has less energy available for animals than grasslands given similar conditons. A way i see around his is that these plants only grow on land, and a lot of area is water covered by microbial mats, providing both grazing areas for herbivoires and water farms of staple food for intelligent species.

The hardest pill to swallow is the precence of all these horrific beasts. Predators use more energy, large animals use more energy. Many species of large predators would require huge amounts of energy compared to other parts of the ecosystem. There is one thing that saves you here - your caves are gigantic. For comparison, a wolf has a hunting area measured in thousands of square kilometers. Your bigger caves can be measured in billions of square kilometers. Even everything being several orders of magnitude less efficient you can probably sustain a minimal population of one or two predator species per cave.

Again, eating plants helps. A grizzly bear is much larger than a wolf, but because is't an omnivoire its territory is not much bigger. Since everything is more spread out, your predators would plabably all have to be pretty mobile to be able to cover their range properly.

Airlocks

The last big thing is how you maintain such a different athomsphere. If cave air is in some way connected to surface air, gases will diffuse and equalise over time. You would need to either be completely cut off, or have very slight exchange with constant production of your own gases. You say dwarves control the shafts of entrances, and the whole complex is built as a bunker? There are airlocks in all shafts, and the dwarfs maintain them.

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The Dwarves are short...much shorter than most people think.

Chemosythetic ecosystems are going to be energy-poor ones, and there isn't much you can do to change that. The Movile Cave has a diverse and complex ecosystem, but that ecosystem is limited to small species. Bigger animals require exponentially larger amounts of energy due to the square-cube law.

So scale down your ecosystem. The "humanoid" dwarves are actually ant-sized. The "horrifying beasts" are the largest of predatory crickets and centipedes. You're going to have to change their physiology somewhat (humans couldn't function scaled down that far), but there are plenty of questions here that can help you with that.

Direct interactions between the surface and the underground are so rare that very, very few surface-dwellers know the truth.

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Yes the use of magic is probably wise to hold the ceiling up if the caves are 500-1000km across. Teleportation drainage for the leaky roof is also a good idea as I doubt there is any other way to keep the water out.

The basic set up other than that sounds plausible but one problem would be a source of light for the ferns. Phosphorescent mushrooms would not provide enough by several orders of magnitude.

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  • $\begingroup$ The tag says "science-based." I don't think "use magic" counts as science-based. $\endgroup$ – puppetsock Jan 15 at 21:10
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    $\begingroup$ The question describes the ceilings as being suspended by magic. I presume magic is allowed in the situation set up. I have often seen handwavium applied to science based questions that use it in the set up but then seek science based answers to the situation presented. $\endgroup$ – Slarty Jan 16 at 11:08

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