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The "Underdark" — as it is referred to in D&D, though equivalents exist in many settings — refers to the massive network of caverns and connected cave systems, ranging in depth from just below the surface to around 20km in some deep parts. I have a similar set of caverns in a setting of my own.

I was just thinking about where the breathable oxygen comes from for its animal and humanoid residents. Such fantasy places usually have rich fungal life and several sentient races. On earth, our first significant O2 in the atmosphere came from the photosynthesis of cyanobacteria, which used the already available CO2 in the atmosphere.

If we assume the raw materials (oxygen stored in minerals, volcanic gases) exist, what sort of lifeforms would be the best candidate for turning the available energy in the Underdark into energy for themselves, with oxygen as a waste product, and where might they prefer to live? If nothing fitting that spot exists today exactly, could that position be filled by a lifeform that only uses traits that exist today in some capacity, even if an amalgam of different species — can a chemosynthetic plantlike structure exist?

For the sake of this question, presume the geothermal gradient is not a substantial issue, because in this setting the entirety of the massive cavern structure known as the Underdark was crafted by divines, and enchanted in such a way that the excess geothermal heat (which was above the level for mortal life to survive in most large caverns) is used up to fuel magic which supports the caverns. Places which have magma by way of volcanic/seismic activity bringing it into the area would still become very hot, though.

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    $\begingroup$ If you're already invoking magic for supporting the caverns and biosphere, why be such a science sticker on oxygen? Just bring it under the magic umbrella too. $\endgroup$ Commented Jun 22 at 3:50
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    $\begingroup$ A common theme is magic forming the base of the trophic pyramid so why not a thing that does photosythesis with magic instead of sunlight. Afterall all those mushrrom forests and purple worms need somthing to form the base of their food chain. $\endgroup$ Commented Jun 22 at 11:55
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    $\begingroup$ @EmberSchott troph is a Greek root so surely it should be thaumatrophic (lit. "nourished by marvels"). $\endgroup$ Commented Jun 22 at 13:52
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    $\begingroup$ Well, THE Underdark has in its deepths a very thin membrane between the planes, so the planar rifts/gates are not uncommon. They are the origin of many monsters living there (and the reason why teleportation magic is ill advised there). Oxygen is easily explained by the presence of rifts to the elemental plane of air. I post this as a comment because the question asks for science-based answers. $\endgroup$ Commented Jun 23 at 12:38
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    $\begingroup$ @Cadence Surely in a meritocracy like ours, only hyperactive sociopaths would mix Greek and Latin roots. I learned this on a television program about ... uh... electrocuting claustrophobic geostationary bicycles. $\endgroup$ Commented Jun 23 at 14:27

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Naturally occurring Electrolysis

So I am going to be stretching things a little - but I wanted an answer that wasn't Pressure differential or plants.

First thought was Thermal Pyrolysis of Water - apparently if you get Water hot enough, it can decompose into Oxygen and Hydrogen. This could potentially work if you had say a super-heated Magma flow meeting a flow of water - but when I did some digging, I couldn't find the specifics (some details said it needed to be done without Oxygen, others said it needed 900+ degrees)

I am including it here in case someone knows more than me and can comment on whether this would work.

Then I started looking at plain old Electrolysis of Water - producing Hydrogen and Oxygen. Only 1.5 volts is needed.

And this got me thinking. Say there is an underwater river system (which is not unreasonable), being deep underground there are lots of Mineral deposits - including say, Native Copper.

Now all we need is some means of generating a consistent Electrical Charge, with Water and on this Copper. I thought of perhaps static Electricity (maybe from Tectonic action or other means) but then I settled on my idea:

Electric Eels!

With enough of them all throughout the cave system, regularly discarging for one reason or another and enough of this Native Copper and fresh water, it produces a steady amount of Oxygen in the Cave system.

But what about the Hydrogen?

Being much lighter, it goes to the top of the cave and escapes very easily and is not a concern.

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    $\begingroup$ Yes, lighter gases escape out. We have it on large cave systems near radioactive ore deposits, where the Helium created by Alpha-particle emissions percolate through the rock and eventually escapes to space. $\endgroup$ Commented Jun 23 at 13:01
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    $\begingroup$ And the Carbon dioxide all sinks to the bottom? $\endgroup$ Commented Jun 24 at 8:54
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    $\begingroup$ "underwater river system" - did you mean "underground" river system? $\endgroup$ Commented Jun 24 at 12:33
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    $\begingroup$ Electric eels still need some energy input to produce their electricity. They will usually consume more oxygen than what can be made via photosynthesis. (Of course, magic can help.) $\endgroup$ Commented Jun 24 at 22:43
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    $\begingroup$ ...and then the hydrogen assembles in an underground upside-down cavity like a dragon cave, and an adventurer lures a dragon in, it mixes up and causes a BOOM hard enough to collapse a city-wide area. Poor dragon, it never knew what hit him. $\endgroup$ Commented Jun 26 at 6:52
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In geology there is something known as a barometric cave or breathing cave. These are caves which react to daily or seasonal differences in outside temperature and pressure. For example, at night cold dense air will sink into the cave and increase the pressure inside. During the day the outside air warms and pressure drops. This pressure difference is enough to cause the higher pressure air in the cave to blow out until equilibrium is reached. When the outside air cools the process repeats.

In this case the Underdark gets its fresh air from the outside through its various entrances to the surface world. As part of a daily or seasonal cycle the air in the Underdark is continually expelled and refreshed creating a sustainable breathable environment.

enter image description here

As a side note I have personally witnessed such a breathing cave when visiting Wupatki National Monument in Arizona. The entrance was far too narrow to climb into but there was enough airflow blowing out to be comparable to a portable fan. Quite a refreshing little anomaly given the high desert environment!

https://musnaz.org/collections/our-collections/anthropology-2/blowholes/

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    $\begingroup$ I love the idea... but I am doubting it would scale well. The Underdark as described by the OP is very, very deep and has some massive caverns, and quite a lot of life. It seems the wind currents necessary would be massive as well. $\endgroup$ Commented Jun 23 at 17:03
  • $\begingroup$ The bigger you are, the slower you breath, so... seasonal windstorms? $\endgroup$ Commented Jun 25 at 4:43
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Given that the tag means that we are not saying that 'Magic is making it habitable', the answer would have to be that while these deep caverns are underground and effectively exist in permanent darkness other than bioluminescence, they are still ventilated by surface winds and temperature variations.

By having cave mouths that catch the prevailing winds and/or act as venturis, the shape of the land-forms and caves forming natural (divinely created) funnels, injectors and ejectors that all serve to force outside air into the caves or suck air out. Temperature differentials from day to night could also serve to help propel air through these large caves.

By these mechanisms, outside, oxygenated air can be circulated within the caves, which would also have been divinely designed to allow proper air circulation and to avoid stagnation.

This is little different to large-scale underground mine ventilation, with natural, divinely-designed ventilation constructs.

So, in the OP's underdark, while there would be no sunlight, there would still necessarily be wind, as well as ways in and out.

As for volcanic areas, the heat generated by magma could heat air, which would rise, pulling in cooler air from elsewhere, and sending it upwards, potentially through venturis that would act as extractors that would pull even more cold air along with them.

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Plants

There is no reason to reinvent the wheel. If your magic system is capable of transforming excess heat into kinetic energy that supports the caverns against gravity, it stands to reason that a similar transformation could create light energy like sunlight. If such energy exists, then it is only a matter of carpeting the whole cavern in moss, which should give enough oxygen to support sparse life. Maybe some areas are lit up but others are pitch dark, getting oxygen from the lighted places.

Mushroom Sun

Alternatively if some quirk of magic prevents it from making light (or you don't feel like handwaving that), chemosynthetic bacteria exist already. Many fungi consume bacteria with their roots, and many also have bioluminescence. This could support small amounts of moss that make oxygen, which is used by the bacteria to make energy which the fungi use to make light which the moss uses to...anyway you get the idea.

Mushroom Cloud

Alternatively if there are not enough minerals to support a population of chemosynthetic bacteria consider radiotrophic fungi, these guys turn radiation into energy which could again be made into light for moss etc. Maybe there was a magic war in the elder days that resulted in a lot of radiation getting released which is now fuel for the underdark, or maybe there is just a lot of uranium and other radioactive waste around (everyone throws their trash from alchemical experiments and whatnot into the underdark, it's big enough to not fill up...right?)

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    $\begingroup$ I read "Mushroom Sun" and immediately I start thinking of Plants vs Zombies... $\endgroup$ Commented Jun 22 at 8:44
  • $\begingroup$ This only works for small case uses like individual caves or regions where perhaps one or more mages have set up some continual lights, the underdark is far more expansive than that and generally depicted as largely natural cave systems and tunnels $\endgroup$ Commented Jun 22 at 16:46
  • $\begingroup$ @Pelinore Not true. The entire system is filled with natural magic, so there's definitely radiant magic sparks floating basically everywhere. $\endgroup$ Commented Jun 23 at 6:46
  • $\begingroup$ @EgorHans no it isn't, not in first edition, nor second iirc, I wouldn't know about later retcons because I stopped indulging the new IP owners semi regular "let's change everything again and make them all buy all new core rule books" cash grab monetisation cycle a long time ago $\endgroup$ Commented Jun 23 at 15:34
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    $\begingroup$ @Pelinore The OP made it quite clear that this is a distinct setting from DnD and that the caves were made by divine means $\endgroup$ Commented Jun 24 at 9:04
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So, if iron is heated, till its red glowing- and you blow steam over it, it splits into hydrogen and rust. Extremely dangerous of course, as the smallest flame would blow this up. Now rust and aluminium- can produce thermite- and thermite gets really hot.

2000–3000°C hot- and at these temperatures- water going through(as steam) can split - into oxygen and hydrogen. How to stabilize this extremely volatile reaction and its byproducts? The problem is, that metal, also binds that oxygen again. And it would be a hellish cesspit, needing constant energy input- more then a simple volcano can contribute. Basically- a lightning strike into the volcano.

Now you need a heat source (lava like)- a source of iron (graveyards, forgotten scrap, natural occurring veins, melting out by the heat) and an interaction of that process. No idea if this is a valid process relative to the energy and metal needed to provide some oxygen. And all it takes is a spark and you are back to water from thin air.

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Life finds a way

Source:

The simplest version is to have some chemical process which consumes something with oxygen (CO2, water, ...) and in the process rejects some amount of O2.

Lifeforms on Earth tend to be built atop H, C, and O, and plants are already using photosynthesis to turn CO2 into O2 as an example... so I'd suggest following along.

Do note that not all oxygen need be released by the lifeform. It's fine if it retains some (or even most) of it for itself.

Energy:

Of course, there's not much light in the Underdark -- that's why it's dark -- however, at least at a certain depth, it should get warm, so heat seems like the energy source of choice for the reaction.

Location:

Water and CO2 are both heavier than O2, so the logical location would be for those lifeforms to inhabit the depths of the caves: water and CO2 would fall down there, and the O2 they emit would rise up.

So what?

In the end, I would think of:

  • Aquatic lifeforms, perhaps some form of algae, or bacteria, which consumes water or the CO2 dissolved in the water, and emit O2 as a result.
  • Mushrooms/Plants, which consume either water or CO2 and emit O2 as a result.

I suppose that consuming oxydized metal ores and such would have a cool factor, but I'm not sure how to explain how said metal gets there, and why there's always more.

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I think one possibility is that there would be plants doing something very similar to photosynthesis, but using other energy sources instead of light as their energy source.

If one considers other dark environments, such as black smokers in the deep sea, it's pretty common to have chemotrophic bacteria, which break down various chemicals coming out of the geothermal vents for energy. These bacteria become the very bottom of their food chain, analogous to plants in almost every other environment. In the deep sea, there is still some oxygen that some of these bacteria may use, but there are other chemotrophic bacteria all around the world, some of which operate in completely anoxic environments (though they are often significantly less efficient at metabolizing their energy sources than their oxygenated parallels)

Similarly, it would not be unreasonable for an environment like the Underdark to have a set of organisms near the bottom of the food chain, or perhaps symbiotic colonies that combine multiple organism types together to provide both food and oxygen for the rest of their ecosystem. For example, lichens are a symbiotic pairing of algae and fungus (sometimes bacteria and fungus), where different organisms play slightly different roles to accomplish a singular goal that helps the whole colony survive. In lichens, the fungus provides the physical structure and environment necessary for the algae to survive, and then the algae provides food energy for the fungus to use. It would be possible in the Underdark to have some sort of colony that combines a plant or fungus and a bacteria, maybe even a fungus too, where you have a plant/fungus that does not photosynthesize, but instead uses existing chemical energy from chemotrophic bacteria to turn CO$_2$ and water into sugars and oxygen, while some of the chemotrophic bacteria probably use the plant for a cozy environment and also use some of the produced oxygen to increase their efficiency at breaking down the minerals or other compounds they use to produce that energy for the plants. It's entirely possible that after a sufficient length of time, just like chloroplasts and mitochondria, these chemotrophs have also been absorbed into the cells of the plants, and it's kind of hard to call them their own organism anymore.

As another possibility, maybe there isn't oxygen, and instead the Underdark uses a different oxidizer. For instance, sulfur is very common in mineral environments (when it's gaseous, it's usually in the form of H$_2$S), and it's still a reasonably effective oxidizer in anoxic environments. Sure, it would make the Underdark inherently very hostile to any life from the overworld (and vice versa. Using H$_2$S to breathe when you're used to oxygen is a bad idea, and so is the reverse), but it would still exist, and it would be hypothetically possible to still have creatures go between them, at least with proper PPE.

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