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All I know so far is, they're a developed culture of humans who live in cave networks underground, made fit for life. They speak their own language, and they have some contact with outsiders, with whom they trade. No one has ever been on the surface, and though concepts of "above" and "heaven" are captured in their songs and stories, no one is interested in "going up". Although, they covet a somewhat rare material outsiders call "lightstone" (you can guess what it does.) They trade with outsiders for lightstone.

They get water from underground sources. Food and fire, I'm not so sure. I don't know if they can grow wood there with only light from lightstone and fire. Maybe some really robust saplings, but could they grow without sunlight?

Without wood, I'm not sure if fire is a possibility - could they just use coal and other non-renewable sources? (If so, I suppose this makes light finite, which is an interesting twist.)

As for food, there are beasts which live in the caves which they can hunt, as well as moss and fungus probably, but beyond that I don't know what they eat, or how they maintain nutrition. They're a hardy people, but there's probably a minimum they need to survive.

My chief concern, though, is how do they live without natural light? Can humans do this for indefinite generations? Would I need to modify their biology to make this aspect work?

So to sum up the chief concerns of this question:

  1. How could a cave-based civilisation grow food?
  2. How can they make fire?
  3. How do humans live without sunlight?
  4. How well do humans live without sunlight?
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  • $\begingroup$ On a brief note, one of the first things we lack when we don't get sunlight is vitamin D, but fatty fish is rich in vitamin D, so if they eat large quantities of fatty fish, which could be cave fish or fishes washed in from underground rivers, they could possibly manage. $\endgroup$ – Martine Votvik Jun 13 '16 at 5:53
  • $\begingroup$ Related: worldbuilding.stackexchange.com/q/394/28 $\endgroup$ – Monica Cellio Jun 13 '16 at 20:30
  • $\begingroup$ This feels pretty broad; you're asking about lack of sunlight, food, and fire, each of which could reasonably stand on its own. $\endgroup$ – Monica Cellio Jun 13 '16 at 20:31
  • $\begingroup$ It could, but they're all strongly thematically linked. It feels like a waste to ask three separate questions for what one broad answer could reasonably address. (On that note, thank you for not casting a close vote.) $\endgroup$ – Lou Jun 13 '16 at 21:38
  • $\begingroup$ Maybe your lightstone gives off the light needed to grow food. We do that in space and in indoor labs, but you'd need soil, etc. etc. $\endgroup$ – Mikey Jun 14 '16 at 8:35
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You haven't specified a tech level, so I'll give a mix of options.

I think one of the simplest ways to do this would be to find some kind of volcanic activity, deep underground. Volcanic intrusion of molten rock into the lower parts of your caves provides heat, and even a little light. You'll want a shaft passing through the layers your people live on and up to the surface (or at least, somewhere it can safely vent large quantities of toxic gas). This carries the volcanic gases away upward to where they won't asphyxiate the people (or you can just have the magma on the other side of a wall, so the toxic gases are kept out - you just need the source of heat). The more important part, though, is that the rising warm air creates a strong flow that draws the air out of the tunnels. This means that fresh air is drawn in at the other end, so you can have this continual flow where fresh air is pulled in at the entrances away from the magma, and then the stale air is drawn up by the rising hot column near to the magma. You can do this with furnaces, but they need fuel which can be hard to come by underground, so living near volcanic activity is an attractive option. This is how termites keep the air in the deepest part of their nests breathable, except that they use the heat of the sun warming their termite mounds rather than furnaces or volcanoes.

Once you have the air situation sorted, you can also use the heat of the magma to allow something like the clusters of life around hot vents on the seabed, and hopefully that can be the bottom of a food chain that eventually produces something edible by humans. Probably from plankton-type things to siphoning sea-sponge type things to crab-like or fish-like things, and from there you're in more recognisable territory for a fish-based food source - kinda like real-world fish or oyster farms, but in vats deep underground. Seaweed gives you good options for "vegetables", and there are algae that have all sorts of beneficial properties. You can also use this for handling waste - sewage, food waste, even cloth and wood once it's been mulched down a bit; basically anything biological. Oysters are great at filtering water, so you can supplement whatever underground streams you're using for drinking water by filtering waste water. The heat of the magma also provides the option of distillation.

If you want, you could probably get some bioluminescent plants (or even creatures) so that you can get low-level lighting just by piping water from the vats through something transparent, and then the water will provide warmth and nutrients (or plankton, or whatever) for the glowing plants to live on. It's probably not bright enough to grow crops, although perhaps with lenses you could focus the light into small patches that could grow a plant or two, which might be enough to grow a few medicinal plants for your doctors. The warmth of the water also lets you spread the heat around and turn some of the cooler caves into places warm enough to live comfortably, so they don't have to cluster so tightly around the magma intrustions and can have a little more living space.

If you have the tech for it, the heat of the magma could be used in some kind of geothermal power (boil water, use the steam to spin turbines and generators, let the steam rise to cooler caves to condense, let the water trickle back down for re-use). That would give you electric lighting, and from there hydroponics and all the rest.

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    $\begingroup$ Great answer! This solves so many of my problems. I may need to do some research, but this is a perfect start. $\endgroup$ – Lou Jun 16 '16 at 19:11
  • $\begingroup$ Bio-luminescent organisms use energy to glow, this comes as food. What are they eating? $\endgroup$ – Donald Hobson Jun 17 '16 at 18:30
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    $\begingroup$ They'd be living in pipes that carry water from the vats, and the vats can be built where the magma is, and they can have waste food and other biological matter mulched and dumped into them. So there'd be nutrients in the water, fed (and warmed) by the volcanic vents in a manner similar to the worms and things that live around hydrothermal vents on the seafloor, and supplemented by mulch. If it's bioluminescent fish, then it's similar but a few steps up the food chain - the nutrients, to plankton, to shrimp (there are glowing shrimp) and then maybe to fish (there are plenty of glowing fish). $\endgroup$ – anaximander Jun 17 '16 at 22:06
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No soil and no sun don't mean no agriculture. IIRC (I can't find the link, though I read about it some time ago), there were several successful experiments using hydroponic technology and artificial light to grow vegetables without sunlight and without soil. They could be using these technologies, maybe thanks to certain kind of "lightstone" that emits the light spectrum that plants need to grow.

Though this is "hi tech" for us, maybe for them it's something that grew out of necessity and after watching the effects of lightstone on some plants.

Two links I've just found, for reference:

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Were their ancestors high tech people? If so, the vitamin D problem goes away with some hand-wavey genetic engineering. After all there are deep sea fish, squid and crustaceans that live their entire lives far below where sunlight reaches and they haven't all become extinct from vitamin D deficiency. So biochemical methods of making vitamin D from scratch without sunlight do exist. Splice in those genes to the ancestors of your cave-dwellers.

Food is your main problem. Cave ecosystems don't produce much biomass. Tiny energy and nutrient input = tiny biomass output. The blind cave fish in an underground lake could be eaten to extinction by one hungry family.

If your folks can set up aquaponics/hydroponics with electric light, as suggested by others, then you can get around some of this. Think giant underground polytunnel or greenhouse-with-LED type farming. For fertilizer, send your folks on expeditions up to bat roosts to harvest bat guano. And some baby bats to spice up their diet!

Warning: they'll need breathing equipment and a good vaccination programme to survive the toxic gases and horrible diseases in the bat cave. BBC wildlife prog on Mexican free-tailed bat cave

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    $\begingroup$ Where does the electricity come from $\endgroup$ – Donald Hobson Jun 17 '16 at 18:28
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There's always magic! In a Game of Thrones telltale game, I believe they have a special tree that burns for a very very long time. You could do something like that- they may not have much wood (or a coal material) but it lasts for a very long time. I'm using magical elements for natural lighting, the caves have huge columns and stalagmites of naturally glowing rock and crystals that have magical origins.

As for food- insects? It has a lot of protein. If there's enough water sources, they can breed and produce fish.

Or as you consider, they can see in the dark and have minimum biological needs for natural light.

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First of all Food:
All plants need sunlight to grow because they have to get the energy from somewhere. Insects and fungi grow in the dark but need plant matter to eat. This does not help much. Using fire to light plants would create a huge amount of heat and use an excessive amount of fuel. So the food really has to come from outside the caves. They could trade for it requiring regular bulk delivery. They could eat roots or things that live of roots requiring them to be near the surface or they could rely on things falling down holes. (very big ones then)
"Glow stones", unless using total magic would be unsuitable as chemical reactions cannot produce a lot of light for long periods of time, there is not enough energy. Also if you want the stones to give off much light they better be translucent or transparent (Although a large surface area helps as well). Given inefficiencies in the batteries don't expect the glow stones to give off more light than putting a battery of the same size in a torch. (and those are carefully designed by chemists. nothing that reactive will form naturally.) (and without the off switch, it'll run down at the same rate whether in use or not)
Someone by now will want to point out that nuclear reactions create far more energy per weight. The radium dial on some clocks is just bright enough to see in the dark. If you want glowing dots on the cave floor to navigate, fine. If you want to clearly see by your glow stones, or even grow plants by them then you have a lethal radiation dose around. Your not going to get this without nuclear refinement facilities either.

In short the answer is dependent on location, what is supplied and tech level, none of which is clear.

continued later ...

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  • $\begingroup$ Okay, thanks for the detailed answer. I was hoping I could find a way of feeding these guys without them relying on outsiders for survival - I want them to be an independent culture distrusting of outsiders. $\endgroup$ – Lou Jun 13 '16 at 21:45
  • $\begingroup$ I'm content to have lightstone/glow-stones be either magic, or nuclear (and the cave people have evolved some seriously rad, by which I mean, radiation-resistant, bodies.) $\endgroup$ – Lou Jun 13 '16 at 21:46
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There is a cave full of life that has been isolated from outside energy sources for millions of years. http://www.bbc.com/earth/story/20150904-the-bizarre-beasts-living-in-romanias-poison-cave https://en.wikipedia.org/wiki/Movile_Cave The explanations is a little dense, but it seems it gets energy from chemical reactions with surrounding rock (slowly eating away at the rock). Humans who figured out how to get energy in a similar way, and who were adapted to eating the food grown in the cave, could get along for quite a long time in a similarly closed ecosystem although it may not be very pleasant.

And of course there is always nuclear energy if they are sufficiently advanced and are living underground near uranium deposits.

  1. The cave based civilization would need to grow the kind of food that grows in caves. Mosses, fungi, etc. They could also raise the beasts they hunt as farm animals.

  2. They can make fire from coal IF they have proper ventilation and exchange of oxygen. They could dry their own poop or the poop of animals and burn it (American Indians used to burn dry buffalo chips). Regardless of the fuel, the bigger problem is getting oxygen in and carbon dioxide out.

  3. Humans live without sunlight be relying on other energy sources to produce the light and heat they need.

  4. Humans live without sunlight ok but depression rates increase. Look at life in the artic circle in places like Finland and Alaska during the winter. Depression increases and suicide rates are high. Vitamin D deficiency can also be a problem since it is produced in the skin in reaction to sunlight.

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  • $\begingroup$ The cave life is small and the air would not be breathable. Eating away rock may not produce enough food to support humans. $\endgroup$ – Donald Hobson Jun 17 '16 at 18:27
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If you're thinking about people adapting to an eternal life in the underground, with time they'd degenerate into blind, pale-skinned brutes with a very low life expectancy. Without proper nourishment and their numbers rapidly thinning in the first stage of their existence in such harsh conditions, soon there would come inbreeding, with all catastrophic consequences that it brings.

IF they are lucky, even living off that little food the caves can offer, they'd become a bunch of cannibalistic idiots.

IF you are thinking of an evolutionary twist right from the start, these cave-people could be descendants of moles or rats, equipped to live as scavengers, able to live off the caves' natural provisons, but unable to develope a 'civilization' as we intend it: Caves are not a proper place to save your data other than on stone. Perhaps they'd be the master in sculpting and graffiti, but either somehow they can truly dig wherever they want and never suface, or their society will be too static to go beyond that of hunter-gatherers.

Their look may vary: for example, if they kept their ancestors' fur, said fur could also hold nutrients for them (yes, there is the yuck factor, but if monkeys can groom each other and eat the results of that work, these creatures could eat their own fungi or body microflora). They'd be mostly blind, but they could see in the infrared and be extremely sensible to vibrations. An omnivore mouth (I imagine they could be cannibalistic so that not one ounce of food get wasted). Long and sharp nose. Large ears, foldable so that during digging they won't get ruined. VERY strong claws and arms to dig. Able to climb even vertical walls.

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  1. Mushrooms don't require light to grow and can provide all the micronutrients that the human body needs. The minirals that the body needs can be provided with what is called blood snow in Antarctica and watermelon snow in Utah. It grows in warm environments but is the most noticeable on snow when it flows with the water runoff. Mushrooms may absorb the fluid of the run off if grown in the medium.

  2. Fire is not required to stay warm. The deeper the tunnels the warmer the walls are from sheer pressure of the soil above it. Some mine are so deep and the pressure so great that the heat is extreme. Look it up online.

3 and 4. I like the answer of glowing creatures to create an underground ecosystem. But if why people are living underground is due to the extreme cold on the surface then bat dung for importing organic material can not be a consideration.

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Wheat grass sprouts don't even need light... actually darkness is better for growing it. It is an excellent source of nutrition and plants like it could fulfill the need for vegetables.

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