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I'd like to have a large underground world. Imagine the "Underdark" which will be familiar with DnD folks. A huge network of large and small caverns connected by winding passageways. The occasional vast underground lake.

I would like for this subterranean world to be populated with wicked denizens. Entire cities of thousands of people (or descendants of renegade elves, dwarves and such from the surface). Plenty of trolls and random monsters, the odd eldritch horror.

There's only one problem... food. It takes a lot of provender to supply an underground empire, after all. And on our Earth, the larder is pretty bare in underground ecosystems. I have a few ideas, but need some more.

  • Way down in the deepest caverns extremophile bacteria are happily eating away at the rock -- after all, this is how the Downbelow grows. This provides a source of biomass slowly trickling up from beneath.

  • There's some kind of critter which is like a cave-salmon. It is born in the caves as a little thing, runs around in the upper world for a few years to fatten up, then along with untold thousands of its kind, races back down to the caves to spawn. Grisly wars are fought over control of the best routes taken by cave-salmon.

  • Um. Well. I hesitate to bring this up... There is a steady trickle of adventurer types from above, and ... um ... well, "meat's meat", y'know.

I'm not sure if these ideas are enough to fuel an overall population of let's say 100,000 human equivalent. So help me out here, my monsters are starving!

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    $\begingroup$ While they require some previously existing organic materiel, have you considered mushrooms? This comment will be deleted for an answer as soon as I come up with more than the word 'mushrooms'. $\endgroup$ – Imperator Aug 27 '17 at 1:37
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    $\begingroup$ They can't be picky alright. Meat's meat. Including rats. Snakes. Bugs. $\endgroup$ – Vylix Aug 27 '17 at 13:26
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    $\begingroup$ In D&D magic or mana is sometimes the base of a food chain instead of light. I remember a similar question about dwarven cities and one idea was mushrooms that fed of ambient magic. $\endgroup$ – John Aug 27 '17 at 15:22
  • $\begingroup$ Obligatory Dr. Strangelove reference. :) youtube.com/watch?v=ybSzoLCCX-Y $\endgroup$ – MichaelK Aug 28 '17 at 13:50
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d_hippo made a good point with "Our foodchain is based on photosynthesis, which requires light."

Your caverns would not be a closed system of caverns. Somewhere it will connect to the outside world, and that's where your supply of energy come from.

  1. Create an intermediary species that lurks just several meters away from the cave opening. This beast will capture animals that stray too near to the cave opening.

  2. Create a species that graze on the mosses that grow in the opening of caves.

    Expanding the idea, get yourself one, or two turtles.

    Turtles!

    Well, not really turtles. Anything designed to grow plants and mosses on its back is fine. Or slugs.

    Green slugs

  3. Now you can create another species that preys on the first two species, or make the inhabitants harvest these two species directly.
    You can even make them raise them as cattle. At no cost, because you can just make the pen open-ended to the outside world without worrying them to run away (they're underground creatures).

  4. As mentioned in 1st point, there will be strays coming in to the cave. Bugs, bats, fishes. Your underground inhabitants can't be picky, food is scarce after all.

    This actually makes a good plot point why the Underdark inhabitants are viewed as ruthless and disgusting: they need to survive in a place where resources are scarce. Competition is not just a competition, it's a death-or-life competition. Only limited denizens can occasionally show up on the surface. And that's make my last point:

  5. Some of those that can go to the outside world (dwarfs and genomes?) can buy supplies - for themselves - and robbed in the way back home.

Update: Olga is making a good point about the diet lacking nutrients but protein. Here is my interpretation:

There are races that are naturally attuned to the Underdark, and there are Surface Dwellers who are exiled into the Underdark. Both have adapted to the diet, but those recently joined them obviously haven't. Those who are not satisfied with the gourmet can harvest plants and herbs and hunt just outside the cave. Of course, that will be temporary, until they accept their fate, or they accept their "fate".

Additionally, they may have a "mushroom farm" on corpses (it's better than eating the cadaver), that stores energy as carbohydrate. However, mushroom is usually low-carb, so you might have to create a new species for that. Animals with plants on their back will also work too!

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    $\begingroup$ Your proposal is interesting but it has one serious drawback. It leads to an abundance of protein sources and lack of everything else. Unless the undergrounders are genetically equipped to handle a diet like this they will be getting sick. Also, it is not the most efficient approach energy wise. If the resources are scarce eating mosses and algae makes more sense, since the conversion of plant calories into meat calories happens at a huge loss. $\endgroup$ – Olga Aug 27 '17 at 15:11
  • $\begingroup$ @Olga fair point. I was assuming the races underground have adapted to this circumstances. I'm updating my question with your excellent suggestion about eating the mosses. As I'm aware of, algae is a source of protein, too, not carbohydrate, isn't it? $\endgroup$ – Vylix Aug 27 '17 at 15:45
  • $\begingroup$ depends on algae, but yes, it is mostly protein. On the bright side, technically you can use algae as the only source of protein, as far as I know. Throw in some potatoes, leafy greens, and some oil, and you should be totally fine. $\endgroup$ – Olga Aug 27 '17 at 16:13
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    $\begingroup$ @Vylix I think you've hit the heart of the solution, which is that we need a stream of biomass streaming into the caves, and by its nature, protein is typically more self-motile than veggies. I can imagine -gout- to be a common ailment down there. Humans can survive on a largely animal diet if they eat -all- of the animal, including the ... hmm ... "ucky parts". (See eskimo example: discovermagazine.com/2004/oct/inuit-paradox ) I imagine salads would be a prized delicacy, the more so the deeper you go, as little orc children wail "Not steak AGAIN!" $\endgroup$ – akaioi Aug 27 '17 at 23:09
  • $\begingroup$ In my understanding, more protein = stronger physique, so when I imagined the whole situation, it makes sense if the inhabitants are either races with strong muscle, or lean ones that exploit the strong by speed (this one does not need too much food). But I don't agree that salad will be a prized delicacy generally. For those adapted to carnivore diet won't stand veggies. $\endgroup$ – Vylix Aug 28 '17 at 4:05
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You can mix and match any of the following:

Scientifically Possible Options

You need a steady flow of either biomass or usable energy to feed you underpeople. These may come from:

  1. Surface excursions - the sentient denizens bring food from the surface by raids or trade. Some cave creatures hunt or graze outside but return to the caves for mating/dwelling/hibernating/raising their young etc. Note that this doesn't have to be a single creature making its way from the deepest cave all the way out - it can be a chain, where every "member" brings the food slightly deeper than the previous one, all the way to the deepest caves of the Downbelow.
  2. Food flows in from the surface - seasonal floods or steady rivers pushing dead material from the surface enable the growth of fungi, which in turn feed your underpeople or some creatures lower in a food chain. Some water bodies may even allow fish and crustaceans to migrate into the caves. (somwhat similar to ZioByte's answer)
  3. Light shaft "oasis" - not all the Downbelow is dark - some rare places do receive some sunlight - either directly through deep chasms and fissures, or indirectly, thanks to the reflecting properties of ice, polished stones, or cleverly placed mirrors. Such places can allow moss and lichen to grow and support a small food chain.
  4. Thermosynthesis - (as Glurth suggested) there are ecosystems on earth based on the heat of deep-sea thermal vents. You may have something similar in deep under water lakes, or even geothermal shafts/volcanic activity as the energy source to support a food chain.

But wait, this is a fantasy setting, isn't it?

Here things can go increasingly creative/crazy - starting with other sources of energy/biomass to feed the Downbelow denizens or support food chains the subside on, and ending with truly fantastic ideas. Used poorly, this is just a bunch of handwaves circumventing the problem. Used in the right measure, this may add a lot of flavor and wonder to your underworld:

  1. Portals - these can provide anything from livestock, plants or sunlight even to the deepest places in the Downbelow.
  2. Magic supported cuisine - some denizens may use magic to grow crops in the dark, duplicate the cattle (or slaves?) so they never run out, or simply conjure creatures for slaughter or magnificent feasts. This magic may come 'out of nothing' or it may be fueled by some resource which is available in the Downbelow (mud, gold, sacrificial virgins, works of art, laughter... anything that works for your narrative)
  3. Divine provision and mythical regeneration - (based on comments by Olga and user54373) Some beings are so powerful that an interaction with them can feed a nation for generations. This may be something simple as manna from the heavens (er, rockceiling?) provided to devoted worshipers, some kind of pact with a force of evil delivering food in return for services or a more dire price. Or even the direct utilization of the fact that some gods and mythical creatures regenerate so well that you can harvest their flesh as a renewable resource - you can have a "meat mine" daily carving into the flesh of Jörmungandr the World Serpent or a Downbelow equivalent of a punished Prometheus.
  4. Spontaneous generation - ever wondered why does The Cave of Spiders without Number never run out of spiders? or did you noted that whenever you break a large geode, small silver snakes slither from it? see those dark red mushrooms? they keep growing year after year where a great hero was once slain... You get the idea, in a fantasy setting, the idea that life can emerge from inorganic matter or just spontaneously appear isn't impossible.
  5. Unusual nourishment - maybe some caves have plants or fungi which feed directly on ambient magic. Maybe some of the denizens are semi-elemental creatures that feed on crystals or metals rather than organic materials, or maybe you have creatures that literally feed on fear (but are perfectly safe to eat!).
  6. No nourishment - and then you have all the fantastic creatures (things?) that just don't feed at all, or at least not as a requirement. They can be golems, undead, elementals, demons, really wired horrors (how does the razor tentacled humongous eyeball even consume anything), people under the direct care of a god, and this list can go on and on. Some creatures just don't care about food in a fantastic setting.
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    $\begingroup$ There is also a divine humanitarian aid :). Please see Exodus 16. $\endgroup$ – Olga Aug 27 '17 at 16:18
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    $\begingroup$ I do like the portal, I do. Now, if it becomes generally known that the death-site of a hero will spawn tasty red mushrooms ... well. I think we have some story elements here. Let's add to this: perhaps a flank of the World Serpent Jörmungandr passes through the Downbelow. The World Serpent is fated to be at Ragnarok, so clearly it's okay if we sneak a couple of filets out of its hide, daily. As long as we're far from its head, we should be okay... $\endgroup$ – akaioi Aug 28 '17 at 7:09
  • $\begingroup$ @user54373 - I never even considered the idea of "meat mines" - they can be grotesque but awesome sites for epics battle, well defended forts, or really exotic bazaars... $\endgroup$ – G0BLiN Aug 28 '17 at 13:18
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You can have an underground "River of Life", which is the underground emissary of a large lake literally choking in Spirulina algae.

The algae will readily die when taken away from light, but they can boot-start your food chain starting with small fishes, following larger fishes, etc.

Large deposits ashore can feed the luminous fungi described by @d_hippo and start some vegetal development.

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Allow me to Quote wikipedia: https://en.wikipedia.org/wiki/Hydrothermal_vent

"The water from the hydrothermal vent is rich in dissolved minerals and supports a large population of chemoautotrophic bacteria. These bacteria use sulfur compounds, particularly hydrogen sulfide, a chemical highly toxic to most known organisms, to produce organic material through the process of chemosynthesis.

The ecosystem so formed is reliant upon the continued existence of the hydrothermal vent field as the primary source of energy, which differs from most surface life on Earth, which is based on solar energy. "

A large underground sea, heavily populated with such vents could theoretically provide the basis for an entire oceanic food-chain, much like plankton does in Earth’s oceans.

The one problem with this scenario is thermodynamic: the oceans must also be cooled, at the same rate the vents heat it, or it would be boil away! One resolution is to ACTUALLY boil SOME of it (in a single location), and let the steam escape to the surface, while an equal mass of fresh surface water pours down (in a different location) from some arctic surface sea.

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I think you have two options: 1) import all your food; 2) grow your food.

1. Import

Ancient Rome or medieval cities can be a good example here. At its peak, the population of Rome was about 1 million people. Obviously, they could not grow their food themselves. So, they were importing almost all of it.

In case of Rome, they had the entire Empire to provide them with grain. You can follow their example and establish a protectorate over the nearby lands and collect tribute in foodstuffs from the surface population. You can design it as a "monster protection fee". To spice things up you can ally trolls and alike with more human-like species in a quest of establishing control over farmers.

Another approach is based on trade (similar to medieval towns and even modern cities). Your underground towns can become centres of free trade where anything can be bought and sold. Your business model should be based on the idea of providing unique goods and services. For example, it can be a place to hire mercenaries, find a rogue mage, or buy unique armour, weapons, and jewellery (elves+dwarves would make a fantastic design team). I would also look into spider silk. If your caves are the only place where it can be produced and made into cloth or armour, you have a monopoly.

With income generated from the trade, your underground city should be able to buy supplies.

2. Food production

This can be tricky. A lot depends on available magic and technology. You also need to think about balancing the diet. If biology of your underground dwellers is similar to human you must provide them with sources of dietary fibre, carbohydrates, protein, fats, and micronutrients (vitamins and minerals). Unless it is a dystopian realm where diet related sickness is a feature.

It might be possible to create something like vertical farms. If your underground citizens can make mirrors, they can create an elaborate system of mirrors reflecting sunlight into caves. This would allow growing food on shelves. Hydroponics (water mist in particular) is the best growing method in this setup, but I am not sure if you can do it with pre-industrial technologies.

In addition to vertical farms for vegetables and grain, you should build mushroom farms for a healthier diet. Insects and worms should also be included to provide protein. Small animals like hares can be raised in cages. They are low maintenance and a good source of meat. However, the mean is extremely lean, so you must provide your citizens with some oil or fat to avoid rabbit starvation. If you have access to the surface in a mountainous region, goats can also be an option. Goats are great for meat, milk, wool, and hides.

As @ZioByte suggests, algae pools can be another source of nutrients, especially protein. They still need light and quite bright for rapid growth. However, algae can feed not just fish but also humans, poultry, and livestock. An additional benefit is that some of them can use diluted human urine as a growth substrate. Hence, you can keep your city clean.

Fish ponds stocked with species similar to tilapia are also a very attractive option. Tilapias grow fast, easily tolerate overcrowdedness, and are very sturdy. While real tilapias are sensitive to water temperature, it might not be the case in your situation.

Pigs would be another great addition to your city. They are omnivorous and can consume almost all food waste produced in the realm. Their excrements can be in turn used to feed algae and shrimp (the latter is done in China).

Throw in some hunting and gathering and your population should be fully covered in terms of dietary needs.

One more thing to consider when it comes to growing food is how centralised the production is. You can have underground farmers or huge government operated factories. Your city might sponsor farming infrastructure (mirrors and irrigation) and then tax farmers (or all citizens) or farmers might be left to their own devices.

Magic makes all of this much easier, especially if your mages can create artificial lights, irrigation systems, and so on. A huge drawback is that they become de facto rulers of your dark realm. Everybody would depend on them for food. Unless they are needed only to set the production up.

A combination of the two approaches would give you the best chances of feeding your population.

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  • $\begingroup$ If you have magic, there's also the very reasonable proposition of plants that naturally feed on ambient magic and don't need sunlight at all. $\endgroup$ – jdunlop Aug 27 '17 at 3:46
  • $\begingroup$ This can be a great story! Plants feeding on magic could be magical themselves. $\endgroup$ – Olga Aug 27 '17 at 15:05
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The main Problem with food in the Underdark is the missing of light. Our foodchain is based on photosynthesis, which requires light. Because the Underdark is, uhm, dark, you'll have trouble establishing a foodchain, because its basis is missing. So you need to address that problem in some way.

One possible solution would be let your world have strong-glowing bioluminescent fungi. So strong-glowing that they enable photosynthesis. They could be cultivated by underdark denizens, who use them to bring light to their farms. Or they could naturally populate larger caverns, who then become something like an underground oasis. Plants could grow there more easily, herds of herbivorous animals would circle between these oasis and monsters would feed on the herds when they travel through their territory. Maybe some species would develop hibernation, to save energy when prey is scarce.

Since your underdarklings don't seem to be on good terms with the surface anyhow, maybe raiding is an option for those in the upperdark. Raiding farms right after harvest could bring in huge quantities of food. Hunting on the surface could also be an option, but i doubt it's practical to feed large underground populations that way; maybe meat, hunted onthe surface, would be considert an expensive luxury.

Edit: As mentioned in the comments, the fungi need a source of energy, too, but I don't see that as a gamebreaker. Fungi don't use photosynthesis themselves, so their energy could come in other ways. They could feed on the stone-eating bacteria mentioned by OP, or on a by-product created by the stone-eating. Or they could feed on organic material from the surface, brought down by water.

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    $\begingroup$ Won't work. Fungi need to get energy from somewhere, even if it's dead wood (to have dead wood you need live trees). You need a non living primary energy source. $\endgroup$ – ZioByte Aug 27 '17 at 12:39
  • $\begingroup$ Biolumnescient fungi don't work unless there is some other power source available to fuel their biolumnescence... in which case, it would be much more efficient to just run the ecosystem off of whatever that power source is directly. $\endgroup$ – Logan R. Kearsley Aug 27 '17 at 14:31
  • $\begingroup$ The main problem is a missing source of energy. Light-producing mushrooms don't solve that problem; sure, they're producing light, but where are they getting the energy for that? $\endgroup$ – Ethan Kaminski Aug 27 '17 at 14:46
  • $\begingroup$ Chemosynthesis. This is probably mentioned elsewhere. You get around needing a source of light if you have chemical energy. This means you need something like a source of hydrogen sulfide - The electrons on H2S have enough energy to allow bacteria etc to fix carbon from CO2, no light needed. To have a source of H2S you might introduce a fissure coming from deep in the ground, it would be smelly and anaerobic, which might be nice for your story. $\endgroup$ – DPT Aug 27 '17 at 16:05
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Food is a form of energy. Therefore, think how the underground ecosystem can gather energy. You have to answer two question:

  1. Where do/did the energy come from ?

  2. How the ecosystem use the energy ?

  3. How the people can increase the efficiency of transform energy to their food ?


Some suggestion:

First question: Where do/did the energy come from ?

  • It is already there for a long time. Dead bodies -> fossil oil ...
  • From surface microbio, wash to the deep by water current (rain water go down) or in the air.
  • From the core of the planet: volcano. (heat is a form of energy, soil, ...)
  • Water current
  • Magnetic field
  • Sunlight (difficult, but possible when there is the hole to the surface)
  • Magic (?!), yes, in fantasy world.

Second question: How the ecosystem use the energy ?

Food chain: Bigger creature feed on smaller creature, then the food chain go on.

The bottom of food chain (the creature that being eat but not eat others) feed on available energy in the deep empire, or the creature itself come from the surface.

The case of feeding on available energy:

  • Some kind of ****synthesis. Replace *** with photo (if there is sunlight), magnet, heat
  • Decompose: fossil oil, coal, any chemical that contain energy.

Third question: How the people can increase the efficiency of transforming energy to their food ?

  • Farming: raise the mass number of creature in the food chain as they prefer. Pick any creature in the food chain, which the citizen can consume their meat or their by-product. Then raise the creature in mass number.

  • Generator: It is great if there are nearly-unlimited power source (water currency, magma pool). The generator turn energy to light for photosynthesis (creature take from the surface), heat or magnetic field to use on particular farm for mass production.

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