Real life vs fantasy underground city

[Conradine]

I'm fascinated with the idea of vast subterranean structures, yet I found little about a topic so specific.
So, could you please summarize what are the problems that make such a structure irrealizable in real life - and what kind of magic would be required to make it work?

[Saintheart]

Air flow, specifically.

[Batcathat]

I'm guessing the lack of sunshine would also be bad for all living inhabitants, whether flora or fauna, not specifically adapted to an underground existance.

[King of Nowhere]

also the fact that you need access to agricultural goods to eat, and those are on the surface.
and waste disposal; before electrical pumps, you pee in a pipe, the liquid goes down, it moves away from you. if you are underground, the liquid has nowhere to go (underground rivers are unreliable, rare, and not much like they are shown in the media; they are generally water seeping through porous rock/sand, and not at all suitable to use as sewers.
and the fact that building underground doesn't really gain much: an underground structure has the same issues of a skyskraper: 10 levels of apartments in a mining shaft and 10 floors on a housing blocks have similar structural issues. and, all things considered, if you are starved for space, going up is easier with real world technology.
if you have technology to fight most of those issues and you are starved for space, though, you also start going down. every major metropolis has a lot of underground structures. from train stations with shops to skyskrapers with 100 floors above ground and 10 below ground, we do, indeed, make subterranean cities.

[TheCount]

Also the structural integrity.
A big cave? It might collapse on you if you work it too much on it's sides.
A cluster of smaller ones? Workable, but it's a chore to connect them.
Also, supporting pillars. You need to think through which you keep or rework or destroy entirely.
It's just too much if a headache if you aren't forced to work on, beside the ones already mentioned

[Batcathat]

It's just too much if a headache if you aren't forced to work on, beside the ones already mentioned

This is a good point. If you want to create an underground city it might be a good idea to start with "why?" (Is the surface too hot? Too cold? Too windy? Too many monsters?) rather than "how?" since it will likely affect how the underground city's made (for example, a city quickly made to fit an entire population fleeing from the surface is likely to look different than one that grew organically over time).

[Conradine]

If you want to create an underground city it might be a good idea to start with "why?"


The main reason I thought is to be undetectable, invisible.
Also, it's much harder to teleport inside a subterranean structure.

[Spiderswims]

Lets see:

1.Air- Once you get a couple feet underground there is a lot less air then being on the surface. And not just air, but fresh air. It is enough of a challenge to simply explore a cave and keep track of the amount of clean air you can breathe. And when you talk about having people living underground, you will run out of clean air fast. People breathe 24 hours a day, so they will use up a limited air supply quick. The big problem here is the people will use up the oxygen in the air soon enough, and the air will be filled with carbon dioxide soon enough.

Air shafts can help, but not all that effectively. You will need a lot of air shafts, and often at least one in each room. so the deeper you go, the more this will be a problem. Plus there is no natural way to move air around to where you need it, until you get to using technology or magic. And this is on top of having shafts for smoke from fires.

2.Water- There is a lot less water underground, unless you live right next to a natural source. But there is a lot less water then on the surface. You need both a clean source of water and a way to get rid of your dirty used water. You will almost always need a surface water source, and most likely something like an aqueduct to get the water to you.

3.Light- It's dark underground. So if you can't see in the dark, you need light. Naturally, this is by fire. But to make a fire you need fuel, air, and lamentation. There is almost nothing to burn underground, so you will have to things brought down from the surface to burn. Then, you need to be careful as fire uses that air your breathing too. And it is all too easy to have a fire that suffocates people in an enclosed place like a cave.

4.Food- There is not much to eat underground. And, on Earth, there is nothing close to an underground ecosystem that can support a city. People need lots of food, and the right kinds of food too. All of that will have to be imported.

5.Nature including Sunlight- Humans at least are made to exist on the surface of the planet, with exposure to nature in general and most of all sunlight. This is a very real problem for a lot of people that live in the north, where they might not see much of the sun for months. And that is on top of them living and working in a pure urban landscape with zero nature. And the health problems here are near endless, like the human eye needs to see large distances, like literally the the horizon of the Earth, for good health. All of these problems and more are problems for the space program.

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So in a fantasy world,

1.Gust of wind, and any other spell that moves air is a must. You would really want magic items, magical architecture. Animated mechanical fans would work great too. Even better are spells that create air and clean air. It is also possible to do some of this with mechanics.

2.Gust of water or any spell that moves water is a must. You would really want magic items, magical architecture. Aqueducts and pumps are a must. As are animated ones. Even better are spells that create water and clean water. It is also possible to do some of this with mechanics.

3.Light is easy enough to make with magic. Plenty of spells make light. And many fantasy races don't need light to see anyway.

4.Magic can create food, so that is a big plus. Also fantasy worlds often have underground ecosystems . For example they have Footshrooms, massive tree sized mushrooms grown for food. And a large number of underground animals.

5.When talking about other fantasy races, they will be naturally made for living underground.

[Ashiel]

I'm fascinated with the idea of vast subterranean structures, yet I found little about a topic so specific.
So, could you please summarize what are the problems that make such a structure irrealizable in real life - and what kind of magic would be required to make it work?

Most of the major problems are solvable with 1st level adepts and a few magic items craftable by 3rd level adepts, which makes it pretty easy to inject into the world.

Create Water and Purify Food & Drink are all you need to make a lot of inhospitable places suddenly open up as possible to settle, and underground is no different. Keeping people fed is a lot easier when you can simply mass recycle spoiled food and tainted water. Purify food & drink even makes toxic and poisonous foods edible, so even if all you have to eat are poisonous mushrooms growing in caves you're covered. As gross as it sounds, it would probably be fairly practical to just collect all the sewage runoff and then purify food and drink it into clean water again.

Create water is actually less useful than purify food and drink for this sort of thing, but it serves a purpose of making drinking and bath water easily available. It also has the side benefit of not risking flooding your caves because the water doesn't last forever so if left un-drank will simply vanish away.

In both cases, the simple addition of these magics can ease the intense burden of creating an underground settlement (or a desert settlement, or a high mountain settlement, or basically anywhere that's generally inhospitable due to poor water and food access).

Beyond that, your next major issue would probably be airflow and lighting. The way this has historically been handled is by digging special air tunnel holes, but with magic and industriousness it could be cranked up to levels of comfortable with spells like prestidigitation to promote airflow and purify the air (as prestidigitation can create breezes and also clean soiled spaces). You really wouldn't even need that many "air machines" either, since well considered placements of them would use the tunnels themselves to channel the airflow, allowing you to space them out to cut down on costs.

Lighting is easily handled with simple candles. If you have a sufficiently high level caster, a called lantern archon can light up a city before its service time is up.

================
As to why you might have an underground city, the simplest reasons I can think of that are environmentally agnostic are mostly concerned with defense. It's legitimately pretty hard to enter an underground city or fortress by any means other than those that were built for entering it. It is exceedingly unlikely that a dragon will be able to raze your city to the ground, and you can't just have raiders riding dire bats or something fly over your walls. It might also be appealing if your settlement is based on some sort of earth-based economy such as mining or raising subterranean creatures as livestock. If you're looking for something more fanciful, the city itself might be guarding an ancient power (for good or ill), or might be granted some sort of blessing by being in close proximity to some sort of magical crystal or stone or something.

Other reasons could simply include hostile weather conditions (such as too hot, too cold, too windy, etc).

Also underground cities might not be completely cut off to the sunlight above. To get some ideas of what some of these near-surface areas might look like, consider glancing at these "underground garden (https://duckduckgo.com/?q=underground+garden&iax=images&ia=images)" and "underground garden fantasy (https://duckduckgo.com/?q=underground+garden+fantasy&iar=images&iax=images&ia=images)" websearches for some neat ideas of what park or garden locations might look like near the surface of the settlement while still being relatively covered or concealed from the outer world.

[Thunder999]

Defense is a big one, burrow speeds are much less common than fly speeds and even rarer are those that work on stone.
In real life walls worked very well at keeping people out, but in your typical DnD setting there's far too many things that can just fly right over them.

[Palanan]

Originally Posted by Spiderswims
Once you get a couple feet underground there is a lot less air then being on the surface.

As it happens, this isn’t usually the case. This page (http://www.eniscuola.net/en/2011/05/13/can-you-breathe-in-a-cave/) has a good discussion of airflow and air quality in caves. Here’s a key section:


Unlike mines, caves are complex systems that generally have more than one entrance and always communicate with the surface through a system of fractures and small conduits. Hence there is appreciable air circulation as a result of the differences in pressure and temperature at the entrances of the caves. The air currents that form inside a cave become more intense when the disparity between the entrances increases, and also when the difference between the internal and external temperatures increases. Often strong air currents at the entrance of a cave are a clue to the possible existence of a karst system. Hence the air inside a cave is continually mixed and there is never the problem of a lack of air or oxygen.

There are special cases, which that page goes on to discuss, but those are exceptions to the general rule that, as the page summarizes, “in caves the air is adequate for breathing and oxygen bottles and gas masks are unnecessary.”


Originally Posted by Spiderswims
There is a lot less water underground….

Cave systems frequently have natural water sources, as this page (https://www.nps.gov/articles/caves-and-aquifers.htm) highlights. In particular:


Karst landscapes often have caves, sinkholes, streams, and springs.

Now, keeping these water sources clean from nearby habitations might be an issue, but underground rivers (https://en.wikipedia.org/wiki/Subterranean_river) are more common than most people realize. Note that the Sistema Sac Actun runs for nearly 150 miles underground.


Originally Posted by Conradine
So, could you please summarize what are the problems that make such a structure irrealizable in real life….

One major issue that hasn’t been addressed yet is the lack of a consistent diurnal cycle. Our bodies are designed around the day/night dichotomy, and they can be thrown off-kilter if that schedule isn’t maintained—as anyone who’s spent 36 hours in airports can tell you.

Several decades ago, in the 50s or 60s, National Geographic ran an article by a cave explorer who lived for several weeks deep underground. He found that without the natural day/night cycle, he completely lost sense of time, and at one point he worked something like 53 hours straight without realizing it.

He also fell into deep stretches of depression and lethargy, so these will be other issues to contend with. People in a community might be less susceptible than someone living in complete isolation, but of course communities bring their own stresses and situations.

[Maat Mons]

If you build your underground city inside a mountain, some things become much easier. You can be pretty deep underground, but still be well above the surface level of some nearby terrain, which helps with drainage issues. And where the terrain is steep enough, you can treat the ground like a wall instead of a ceiling, and put some windows in. This is how I tend to imagine dwarven settlements.

Another approach is to kind of just build a house, and then pile a bunch of dirt on top. You now live underneath a hill, which provides some decent insulation and camouflage. Or, more likely, you start with a small hill, dig away parts of it build something half underground, and then pile the excavated dirt on top so you're nor full underground. Either way, your first floor is on the same level as the surrounding terrain, and people who get close enough are surprised to find a door and some windows going into a hillside. This is how I imagine gnomish settlements.

Since I've talked about dwarves and gnomes, let's talk about elves, even though most of them don't live underground. You shouldn't discount the defensive benefits of building in a forest. With an unbroken canopy overhead, enemies flying above won't be able to see that there's a town at all. And the surrounding forest makes it difficult to march large formations through and nigh impossible to move in large siege equipment. While at the same time, providing excellent cover for your guerillas, commandos, and skirmishers. A red dragon could still wreck your day by breathing fire from above, but other than that, large flying creatures can't actually get through the tangled mess of branches above your home to get at your people.

[Fuzzy McCoy]

You can build vast subterranean structures - just look at the Wieliczka salt mine (https://en.m.wikipedia.org/wiki/Wieliczka_Salt_Mine), for instance. One of the big problems they have is flooding, so lack of water is definitely not a problem either, though it would have to be desalinated to be usable.

[Conradine]

About the food...

in real world, does edible algae or seaweeds able to grow without sunlight exist?
Also, edible insects and fungi?

And, blind fish, crustaceans, worms?

( I guess that the mini-ecosistem of Redcloak and Xykon prison cannot be replicated )


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Another thing, it's possible to compensate lack of sunlight with a special diet?


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Aaaaand...
phosphorescent fungi for illumination?

[Batcathat]

in real world, does edible algae or seaweeds able to grow without sunlight exist?

I'm not expert but since there's life in the really deep (and completely dark) parts of the ocean I'm guessing it's possible.

[Segev]

Interesting thought that would lead to more "alien" dwarves and other subterranian creatures: Maybe they breathe CO2 the way plants in their day-cycle do, and exhale O2 as a byproduct (again, like plants). They actually use torches and such underground not just for light (which, in 3e and 5e, they do have a use for despite darkvision), but to "refresh" the air they breathe.

Dwarves and gnomes visiting the surface actually find the air "thin" because the CO2 concentrations are lower than in their mines where certain creatures and their fires do produce CO2, and dwarven respiration cycles it back out.

[Tiktakkat]

How vast?
How subterranean?
How much magic?
What kind of creatures?

A lot of factors have been addressed. One that I note missing is temperature.
Caverns near the surface tend to run a bit cool - 55 degrees Fahrenheit or so.
Deep mines, 1000 feet or more deep, can run rather hot - 100-130 degree Fahrenheit.
Simple fireplaces can handle the cool temperatures, but it will require adaptation or magic to handle the hot temperatures.

For humans, living in areas carved from rock is not impossible, and has been done from earliest recorded history.
Building big enough mines deep enough down for temperature to become an issue took longer, but has still been done since pre-Roman times.
So those elements can be overcome.

For air and water, micro-gates to the elemental planes or trapped elementals can handle those. For lower magic, spells like Darsson's Cooling Breeze, Chilling Chamber, and Fiery Furnace should cover most air and climate requirements if the local cavern is not sufficiently hospitable.

Light can be subjective if whoever is living underground has darkvision and does not need to write. If light for writing is required, there are a number of available spells, both temporary and more permanent. And then of course you can go the Vault of the Drow route, and have a form of magically radioactive mineral decaying and serving as a pseudo-moon, light included. In Forgotten Realms, this is "faerzress". Other settings can have their own magical macguffins.

Food should be the biggest issue, but all settings manage to handwave that with super mushrooms and special underground beasts.
And of course various non-humans can have different requirements.

Construction should be an issue. Disintegrate solves virtually all the excavation and waste issues. Stone shape can resolve support structure issues.

At this point, I will note the major flaw in relying on magic, that being dispel magic. A bit of excess dispelling and everything collapses. That means handwaving in high level casters to create the city, likely backed by special reinforcements and restrictions on dispelling.

[Palanan]

Originally Posted by Conradine
in real world, does edible algae or seaweeds able to grow without sunlight exist?

Algae and seaweeds (which are also algae) are photoautotrophs and completely dependent on sunlight. This is why kelp forests usually don’t occur below 100-200’, because beneath that depth sunlight doesn’t penetrate with enough energy to support them. This is also why deep-sea corals don't have symbiotic algae (zooxanthallae) the way shallow-water corals usually do.


Originally Posted by Conradine
Also, edible insects and fungi?

And, blind fish, crustaceans, worms?

Cave fish probably don’t occur in commercially significant numbers, so not likely to support even a small human community. Most cave ecosystems are dependent on detritus from outside, so while there are some unique and fascinating species, their biomass likely isn’t enough to keep a person going.

As for fungi, there are many species that grow in caves, and some species will colonize bat guano, so that's one possibility for a food source--though not one many people would be too enthusiastic about.


Originally Posted by Batcathat
I'm not expert but since there's life in the really deep (and completely dark) parts of the ocean I'm guessing it's possible.

If you’re thinking of the rift-vent communities, these are based on chemoautotrophs, which are bacteria able to generate energy from inorganic compounds--in this case the hydrogen sulfide which suffuses the rift environment. These bacteria either live in symbiotic relationships with other species (famously Riftia pachyptila) or grow in bacterial mats which are grazed on by other creatures.

Otherwise life in the deep ocean is dependent on “marine snow,” which is organic detritus filtering down, as well as major events like whalefalls. These can support a lot of weird creatures, such as Osedax, which are polychaete worms that feed on whale bones.

.

[Batcathat]

If you’re thinking of the rift-vent communities, these are based on chemoautotrophs, which are bacteria able to generate energy from inorganic compounds--in this case the hydrogen sulfide which suffuses the rift environment. These bacteria either live in symbiotic relationships with other species (famously Riftia pachyptila) or grow in bacterial mats which are grazed on by other creatures.

Otherwise life in the deep ocean is dependent on “marine snow,” which is organic detritus filtering down, as well as major events like whalefalls. These can support a lot of weird creatures, such as Osedax, which are polychaete worms that feed on whale bones.

Yes, I believe rift-vent communities are at least vaugly what I was thinking about. I didn't know about the marine snow but I suppose it makes sense. "Whalefall" might be one of my new favorite words.


.

On a unrelated note, why was there an invisible dot at the end of your post?

[Maat Mons]

I'm pretty sure that converting CO2 into O2 requires an input of energy. I think that's why plants need sunlight to do it. So plant-like dwarves and gnomes would need something to replace sunlight as an energy source. I remember reading about some microorganisms that can do something akin to photosynthesis with scarier forms of radiation. That's how I've always imagined the faerzress-based ecosystem was handling things, actually.

There are lifeforms that don't need oxygen at all. I mean, they don't produce energy as efficiently as lifeforms that do use oxygen, but they exist. And there are even a few aerobic (oxygen-using) lifeforms that can switch over to an anaerobic (not-oxygen-using) mode for pretty substantial lengths of time. A notable example is carp, which can reportedly go months without oxygen, under the right conditions.

If your subterranean city is built around a profitable resource, you might just be able to pay to have food shipped in.

[PoeticallyPsyco]

About the food...

in real world, does edible algae or seaweeds able to grow without sunlight exist?
Also, edible insects and fungi?

And, blind fish, crustaceans, worms?

Not plants, no, but the bottom of the ocean mostly consists of creatures that feed on decaying organic matter that drifts downwards from the surface like snow; these are the bottom of the food chain, and other creatures of course prey on them, and larger creatures on those creatures and so on (and the occasional large corpse drops all the way down, creating a feast that draws creatures from miles around). This makes them still indirectly dependent on sunlight, but there are two ecosystems under the waves that are completely light independent. The first is volcanic vents, where tubeworms draw nourishment directly from the chemicals being released by the vents. The second is a mini-'lake' of heavier than water liquid, again with chemicals that some creatures can draw nourishment from without needing sunlight at all.

On the subject of air-providing items, Stormwrack has some pipes that pump fresh air from the surface.

[mabriss lethe]

The cheapest, low level way to permanently light an underground city is with a level 1 shadowcaster with the Liquid Night fundamental. Create an ink that gives off the same light as a candle, can be made at no cost, and is only destroyed by washing it off. doesn't give off heat or burn O2.

[Tvtyrant]

I'm fascinated with the idea of vast subterranean structures, yet I found little about a topic so specific.
So, could you please summarize what are the problems that make such a structure irrealizable in real life - and what kind of magic would be required to make it work?

In real life there is boiling hot water pressed into the rock once you get down far enough, full of methane eating bacteria (https://en.wikipedia.org/wiki/Deep_biosphere). The heat and pressure are too great for complex organisms, but you could have cold seeps further up. I have run dwarves based on deep sea vents before where they eat tube worms and "dwarf ale" is actually a mixture to help them digest methane based life.

Also the pressure is too high for caverns after a mile or so, which means the underdark is going to be pretty shallow.

[Palanan]

Originally Posted by PoeticallyPsyco
The first is volcanic vents, where tubeworms draw nourishment directly from the chemicals being released by the vents.

Not directly. As I noted above, the tubeworms have internal colonies of hydrogen-fixing bacteria that are the essential first step in bringing chemical energy into the rift-vent communities. Without the bacteria none of the other species could survive there.


Originally Posted by PoeticallyPsyco
The second is a mini-'lake' of heavier than water liquid, again with chemicals that some creatures can draw nourishment from without needing sunlight at all.

These are cold seeps, which often occur in brine pools, which are saltier and thus denser than the surrounding seawater.

Note that cold seeps aren’t necessarily colder than their environment; they’re “cold” in comparison to the hydrothermal vents, where water is superheated when discharged through cracks in the seafloor.

[Rawrawrawr]

About the food...

in real world, does edible algae or seaweeds able to grow without sunlight exist?
Also, edible insects and fungi?

And, blind fish, crustaceans, worms?

( I guess that the mini-ecosistem of Redcloak and Xykon prison cannot be replicated )


----

Another thing, it's possible to compensate lack of sunlight with a special diet?


----


Aaaaand...
phosphorescent fungi for illumination?

Okay, so if you really want something scientifically accurate, the biggest thing life needs is energy - the reason why so much life exists close to surface on earth, with easy access to solar energy, and less so deeper underground or in deep oceans. And you may not think of it this way, but most life uses solar power to survive:

For example, you can think of plants as solar-powered batteries that store energy as organic material. When an herbivore eats that plant, it gets that energy - so it's essentially using days, months, or years of solar energy that the plant has stored. When a carnivore eats that herbivore, it's getting the cumulative solar energy of all the plants that that herbivore has eaten recently - it all boils down to solar energy

But wait, there's more! Big plants can grow in nutrient-rich soil, but that soil is nutrient-rich because other plants and animals have died there. All that solar energy that those things had absorbed, directly or indirectly? Now up for grabs again. So a 50-year old tree doesn't just store 50 years of solar energy, it also stores the solar energy of decades of life before it - which means that anything that eats that gains that energy, and so on and so forth.

Even most deep sea life basically relies on this - basically, life on the surface takes in solar energy, dies, and then drifts down to the depths, making that solar energy accessible in places where sunlight wouldn't normally reach.

And of course, sometimes life can find other sources of energy - deep-sea geothermal vents are the classic example of life that doesn't rely on sunlight, and absorbs geothermal energy from the earth instead (but it's the same basic idea - some life absorbs geothermal energy directly, and then some other life eats those creatures to gain the geothermal energy they've absorbed, etc.).

So basically, if you want "scientifically accurate" life/agriculture/etc. underground, you have two options:
1. You need an alternative source of energy that plants absorb instead of sunlight, like geothermal energy or magical energy or whatever. But it needs to be plentiful, and to have been around for a while.
2. You need someway to "transport" solar energy underground - basically, massive amounts of plant and animal life that have mostly grown up on the surface, need to find their way underground, all the time, dead or alive.
The more energy available, the more life there is, and the more complex it can be.

There is also one last side-note - weirdly, life tends to grow bigger in energy-poor environments, like the deep sea or below ice - so scientifically speaking, in a magical world the deepest caves probably do have the biggest, nastiest monsters :smallwink:

[Khedrac]

Also see Derinkuyu (https://en.wikipedia.org/wiki/Derinkuyu_underground_city) for a real-world underground city.

They have happened, but they are far from ideal for the reasons given so you need really strong reasons to build them.