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In this setting, people are living in isolation in what is effectively a dungeon. Dungeons are usually built with an ecology in mind. Standard answers to common questions such as "Where do clean air and water come from?" are usually solved through use of magic. I'm looking for solutions that are a bit more science-fantasy.

If there are any existing settings/RPGs that answer this question, that would be a valid answer too. I'm aware of Earthdawn, which has a set-up in this vein, but that's about it.

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3 Answers 3

Since your setting is effectively a dungeon:

Clean Air

There are air tubes installed. These are shunts from the subterranean levels all the way to the surface. Some carry fresh air in. Others carry waste air out. Giant fans (mechanical, magical, or otherwise) control the vital flow of air.

Plot Hooks:

  • Finding a larger shaft could be the event that leads to discovering the subterranean area.
  • An important shaft gets blocked. Characters must investigate and fix. Could be a cave-in or a monster or another civilization.
  • Water or some other threat starts to pour down a shaft (or all of them). Why?

Clean Water

There are numerous ways to handle this. The easiest is to have a natural water spring (or even an underground river) flowing through the rock. Clean water has two halves: fresh water in, waste water out. An underground river can flush away sewage and waste (someone else's problem).

Water can be condensed from air, especially in a science fantasy setting. Recall the "moisture vaporators" in Star Wars: A New Hope.

Rain water will probably come down air ventilation shafts, too. People can collect it and ration it.

You can also make water part of the economy. Maybe someone else has the water and they bring it in huge tanks to the people for barter.

Plot Hooks:

  • Something stops the flow of water. A faction is damming the river upstream to create a fishing lake.
  • Suddenly the water is undrinkable (or unpleasant) due to chemicals. Perhaps this is a sign of dangerous seismic activity.
  • The water flow increases dramatically, and threatens to flood the area.

Heat

Two miles underground, the temperature is going to be around 160F (70C). Let's assume these subterranean cities are nowhere near that deep. In general, you can assume that it's not cold down there. If it's deep, you might need a way to cool things. Again, water pumps and air pumps seem like viable solutions.

Plot Hooks:

  • The temperature starts rising. Volcanic activity is on the rise, and the citizens must hurry to cool things down.

Food

There's no natural light down there. You're not going to have green plants, and that collapses the ecosystem quite a bit. Still, life has a way. There are numerous types of subterranean fauna. Feeding a large population is going to be tricky. You might have to hand-wave vitamin deficiency.

Maybe the food comes from the outside through drops or barter.

If you want an isolated ecosystem, maybe there are machines that filter algae from the underground river. The people make it into all kinds of stuff, none terribly pleasant, but all fairly nutritious. Maybe there are some kind of subterranean crustaceans you can farm.

Plot Hooks:

  • A new kind of creature is prowling the caves or the river, eating your food. Kill it or starve.
  • Suddenly the river is teeming with blind fish! Everyone scrambles to harvest this bounty and make a profit.
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1  
You can grow mushrooms. Ants do it. –  Stefano Borini Sep 10 '10 at 8:54
    
sorry, but if you use air shunts to get clean air, and get food from outside through barter, the questions are: "How did they build them? (They could hardly have started a civilization without them)", and "Why are they living underground in the first place?" –  Adriano Varoli Piazza Nov 24 '10 at 23:02
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@Adriano: there may be a purification plant on the surface (maybe the civilization was both above and below ground at one point, but the surface-dwellers were killed off somehow), or the vents may be located in an area where living quarters can't be established. Maybe the planet's ecology changed, and the land-based inhabitants suddenly found themselves on a planet with 90% surface water. Maybe this is a test for an interplanetary colonization effort, and over time, people forgot what the test was. –  Dave DuPlantis May 18 '11 at 15:31
    
RE Plants, and the lack thereof, maybe there's mirror setups to direct light down narrow shafts and power small farms? –  Dakeyras Jan 14 '13 at 16:28

You'll need some source of power, and some source of oxygen.

The O2 can be justified by some form of sealed fusion system turning water into helium and oxygen (Crack it for the H2, release the O2, fuse the H2 into He, Li, C and Be...). Or it could be by rock-eating semi-anaerobic bacteria (IE: Oxygen tolerant anaerobic strains), circulated by convection systems. Or it could be from the agricultural zone.

Water will be sealed cycle, unless you have a fusion plant.

Power could come from any number of sources, including geothermal, solar boilers, light conduits, radiothermal generators, sealed fusion or fission systems, bacterial battery stacks, or even Zero-Point Energy cells¹.

Water, food, and air are in fact going to be primarily functions of power distribution; if you can grow photosynthetic plants, you have food and air, plus recycling of organic wastes. The plants will give off water vapor, which can be drawn out and condensed elsewhere as pure water, and can be fed compost which has broken down the organic wastes of the animal load.

One way of getting light for those plants without electricity is the fiber-optic cable.... a thick (0.5m or even 1m) fiber cable cemented inside a conduit, leading from the surface down to growth chambers; the cable could be fed using mirrors, even, so the light is really bright.... such systems might even have redistribution nodes....


¹ ZPE cells are part of some theories behind Hawking Radiation and some other odd stuff. They are not generally accepted as plausible.

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I'd like to add some further comment / idea to the other (good) answers:

  • Active carbon filters are a reasonably accessible technology that can provide good air and water filters. Maybe you want a society where all the bones of the people are converted in carbon filters, so they have some kind of special rituals and respect of the dead, as carbon filters made from bones are particularly good (bones are high porosity and controlled carbonization turns them into high quality active carbon) and guarantee the survival of the settlement (think about body water in Dune and substitute water with bones).

  • You can have mushrooms and bacterial or algal growth underground, but they still requre moisture, some soil, etc.

  • You can have veggies or algal growth under UV lamps, to provide some kind of nutrients (think about hydroponic cultures and similar, or about plants that are grown in closed cupboards...)

  • You need some sort of source of energy, probably coal or oil, given that you are underground. Having that source of energy diminishing and approaching exaustion can be a major challenge for the settlement, that will be thrown in the doubt: saving energy and staying in the same place, or trying to find new sources of energy by moving somewhere else (that is very energy consuming and risky)?

  • Psychology / psychiatry could be interesting in this environment: do settlers get depressed and psycho by the lack of sun or do they feel protected in their underground settlement and would feel "exposed" on the surface (as in Asimov's Caves of Steel serie of novels)? Or both???

  • There is a Philip K Dick book, The Penultimate Truth, about an underground city-factory building robots for a war on the surface, where radiation due to the war supposedly made life impossible. In reality the war finished long time ago and on the surface there is an elite of former generals that play war and politics with each other by using the robots made by the underground city-factories, that are constantly under strain due to increasing quotas required, while being controlled by global medias proclaiming that the war is still raging on furiously and more robots are needed to win... maybe you want to have a look at it first...;-)

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