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Related to my previous question : How to live in an underground city?

The refugees managed to live in their underground city with the few resources they can find. They are prepared to fight if necessary. But they do not want to fight if they can avoid it. They are afraid. If the army does not find them, they might avoid fighting!

Things you might need to know :

  • The place where they are building the city is in the middle of nowhere. A land full of rock with a small river and some grass on top. Before their arrival, the underground network of galleries looked like a few holes in the ground from the outside.
  • They can use some magic to build or move rocks (nothing more than what medieval technology could do, but faster)
  • They need to go outside quite often (I'm afraid this will make their paths hard to hide)
  • They have children and animals (sheep for example) and perfect silence seems impossible.

How to hide the city : my currents ideas

  • Make the path from the army to the city very difficult to traverse: piles of rock in the tight passages, traps (what kind ?), guerrilla. But my refugees are not warriors, and the enemies are. The more they avoid contact, the better.

  • Watchmen, of course, will give an alert when the army approaches, so emergency actions are possible. But what actions can they take aside from hiding everyone in the deepest, darkest places?

  • Restrain or close holes/entries. That will make life in the city less comfortable, but easier to hide.

  • They can redirect the smoke from their fires to hide it with chimneys.

How can they hide the underground city and the marks of their activities ?

I doubt they can hide it perfectly: what will betray them ?

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  • $\begingroup$ Where are going when they go outside frequently? $\endgroup$ – Green Sep 19 '15 at 2:59
  • $\begingroup$ Watchmen, shepherds (Sheep can be hidden in the city, but there is grass outside, it's easier to let them feed outside when there is no danger), and patrols looking for other refugees, at least. Plus, they could find a part of their resources outside. As they are not used to hide and defend themselves at first, I think normal people will go outside to have a bit of daylight from time to time, until they properly prevent it if necessary. $\endgroup$ – Tyrabel Sep 19 '15 at 6:28
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A perfectly hidden underground city is one that is completely self-contained in terms of food, water, air, and light. Medieval cooking will require fire of some kind so we need to work that out too. No ingress or egress is needed for a self-contained city. Since this kind of completely self-contained city isn't possible (or desirable for this story), let's walk through each need and pick some best methods to conceal.

Assumptions

Limestone formation

  • There's a significant cliff nearby with passages between the galleries and the cliff face.

Air

Given that the area is already perforated with galleries and holes in rock faces getting air in and out of the galleries shouldn't be difficult. Adding many extra air shafts through relatively soft limestone shouldn't be too difficult, especially with magic. Concealing air shafts shouldn't be difficult.

Light

Darkness in a cave is absolute. No one wants to be constantly lighting lanterns for feeble flickering light so some kind of natural lighting will be needed. Natural light is free and generates no smoke. Methods for concealing air vents should work just as well for light.

Sinkhole entrance

Ingress/Egress

Sometimes the entrances to a limestone cave system can be enormous at hundreds of feet wide. However, they can be quite small. The original entrance to Luray Caverns in Virginia, USA was too small to admit a human.

Small entrances make convenient choke points and are easily hidden. Large sinkhole style entrances are also difficult to find unless someone essentially falls into them. Surrounded by dense vegetation, an intruder really will need to fall into them to find them.

Large sinkhole entrances, as see in the light picture can be quite high requiring special equipment to rappel down to the cave floor. This improves defendability considerably.

Small entrance

Water

Water in a limestone cave is never ever a problem as limestone caves are formed by water seepage over millions of years. Depending on where the cave citizens collect it from, it won't need any purifying or boiling. It may taste funny though. (Perhaps they will invent steam distillation to get rid of any funny taste.) No need to conceal this.

Food, Fuel and other consumables

Food is the one resource that can't be had in cave in any kind of quantity. The kinds of foods that humans prefer don't usually grow in caves (yes, mushrooms grow in the dark but mushrooms alone aren't going to sustain a population.) Nearby fields for farming will need to be concealed as well by vegetation or having the field(s) be considerably higher or lower than the surrounding terrain. You don't want the enemy commander to just look out over the surrounding area and see neat little fields that don't belong to anyone.

These refugees need fuel for cooking and heating. Unless this is a cave in a tropical or hot dessert climate, the ambient air temperature in the caves will be the annual average topside temperature. If the ambient cave temperature is in the 40s or 50s (implying an outside climate that drops below freezing in the winter), the refugees may have to constantly fight hypothermia. Fires for heating will consume fuel at a prodigious rate and generate lots of smoke that will need to be managed (see next section).

Smoke

Smoke from cooking fires can be smelled from long distances. And just as predators seek prey by smell, so too will the enemy army. Smoke from fires will need to be dispersed below detection thresholds or completely captured.

  • Capture the smoke with magic, if the magic works that way.
  • Run the smoke through a multiple stage filtration system. Since there is running water nearby, force the smoke to come up through the water before being vented to the outside. Water soluble aerosols will stick in the water. It's really important to do this kind of filtration through running water. Really really important.

Sound

Given an underground city, it's possible that the enemy troops may walk over you on their way to someplace else. While the air and light shafts may be easily concealed, the sound of people living in the galleries will need to be carefully managed. It would be unfortunate for all other concealment measures to work perfectly until a baby starts crying and gives away the game.

Long baffles or plant fiber blankets on the walls or lining the air/light shafts (where possible) might do the trick.

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  • $\begingroup$ Great answer, thanks. I don't have lots of wizards and they are not going to filter smoke, but I appreciate the filtration system. I'm still waiting for other ideas, but you get the point for now :) $\endgroup$ – Tyrabel Sep 20 '15 at 8:25
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    $\begingroup$ Ha! Just thought about sound proofing too. This was a fun question. Thank you. $\endgroup$ – Green Sep 21 '15 at 15:19
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If your enemy is looking for you, then you can't hide. He'll find you eventually. Because of this, your first concern should be:

Secrecy

If your enemy doesn't know your city exists, he won't look for it. And if he does find it, it will be by accident. Your enemy will not be prepared to fight, and therefore can be eliminated quite easily by your troops, even if they're not trained soldiers, as long as they're equipped for the task.

Of course, you don't have to kill the enemy. You can simply keep them imprisoned or somehow coerce them into staying with you. Naturally, this opens up the possibility of a traitor or spy hiding amongst your "immigrants", but can also make for a great introduction to your city and people. By introducing an outsider, you can explain the way your society works in a natural way.


Of course, secrecy can not be kept indefinitely. Eventually rumors will pop up and your enemy will send out scouts. With a little bit of luck, one of those scouts will return with the Holy Grail, Excalibur, water from the Fountain of Youth, etc.

With people coming and leaving, you'll need to make sure the enemy doesn't follow them back to the city, but how?

Decoy

Now that your enemy knows you're there... somewhere, you won't be able to cover your tracks. Well, don't do it then! Start building roads, lots of them. If your roads connect two or more cities, leading right in front of yours on the way, who'd think to look there of all places to find the rumored Hidden City? That would be too obvious! The Hidden City must be hidden somewhere in the forest to the north, it can't possibly be on the roadside.

To further throw your enemies off, add fuel to the fire. Now that your city is the subject of rumors, let's create more rumors!

  • The Hidden City is inhabited by forest fairies.
  • Its entrance only appears during the night.
  • It is guarded by two ferocious humanoid wolves.

And so on. Anything to make your enemy believe your city can't possibly be hidden in plain sight. If you're good at it, the rumors will spin out of control and might appear so ridiculous, that people will stop believing in them. And if people don't believe in this obviously ludicrous Hidden City, then they'll eventually stop looking for it altogether.

Additionally, your citizens could easily travel to one city or another without attracting much attention. The people from City A will think they're from City B, and vice versa. The more cities your roads connect, the more difficult it will be to guess who these travelers are really from.


This leaves us to the question: How do we hide that city from all the travelers and merchants who will use your roads, and how can we ensure nobody accidentally stumbles upon your city?

Camouflage

This part depends a lot on where the entrance to your city lies. If it's a cave by the mountains, you could use a boulder as a gate using your magic. A boulder in front of some rocky cliff would be as out-of-place as a tree in a forest; that is, not at all. If the entrance is a hole in the ground, you could bury it, then unbury it whenever someone needs to go in or out.

Of course, the above methods assume you have mages stationed at the entrance who can open or close it fast enough for it to be practical. For a non-magical yet still practical solution you could use bushes to cover the entrance. This can be used easily without magic, but might require periodic maintenance. You wouldn't want the bush to suddenly dry up and/or die out, leaving your entrance plain for everyone to see, would you?


Of course, you can't allow the kids to go outside. Unlike adults, they're more likely to catch attention. What are kids doing in the middle of nowhere? Are they travelling between two cities by themselves, without their parents or any other adult? Suspicious...

They're also much less cautious, and might lead strangers into your city, knowingly or unknowingly. Maybe they'll mess up with your Guardian Bush, breaking off a few twigs here and there. We can't have that. The kids must learn to behave!

And the animals are going to pose a problem too. If you're lucky, you'll have a pasture surrounded by mountains, and the only way to access it — aside from wandering the frozen peaks — would be through the tunnels of your city. Outsiders discovering that pasture would be very unlikely.

Unfortunately, life isn't as easy as that. You'll have to keep your herd and cattle outside, in an unprotected pasture. This would inevitably lead to the question: whose cattle is this? And of course, there's the predators and the eventuality of raids. You'll have to protect them somehow, and that could easily jeopardize the hidden nature of your city. It might be best for your people to change their eating habits. No need to worry, though. You don't need pastures to raise pigs or chickens. I hope your people like bacon & eggs.


Finally, what would be the biggest threat to your city? People. Many of them.

The more people leave or enter the city at once, the greater the risk that someone will notice it. You'll have to ensure that doesn't happen. Also, if your people want to assemble, they'll have to do it within the city, or reasonably far away from the entrance.

If you can, you should also try and redirect the river to flow through your city. It'd be best to avoid having people leave and enter if possible. If they can get their water directly from within the city, don't have them pointlessly exit and reenter to do it.

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  • $\begingroup$ They're going to trade their sheep for pigs and chicken, thanks for bacon and eggs ! $\endgroup$ – Tyrabel Sep 20 '15 at 8:20
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Listed below are your primary concerns and some suggestions about them.

Smoke

Smoke is the biggest and loudest giveaway of your position. Smoke is visible from far and is a dead right clue that humans are present at exact that point. The amount of smoke would also hint on the size of human settlement. Since you have a complete city underground, I'm afraid the amount of smoke would be voluminous!

First things first. Limit all fires to night time. And then too, there should be no fires lit by the public for more than 1 hour. In this one hour, they should cook, take bath (in winter), heat water for cloth-washing ... whatever. NO SMOKE FORE MORE THAN ONE HOUR AT NIGHT. PERIOD. If that one hour is late night, all the better.

Food

Since your people are keeping livestock, and they livestock has to be taken out for grazing, you will have to develop a routine for that. The side your people use for taking the sheep/cattle out of the catacombs should be paved completely with water-stones and twigs. This would ensure that there are no footprints for a curious eye.

Water Stones

Furthermore, if your people hunt food to supplement their diet, do not use arrows. Lost/missed arrows stick into things and stay there prominently for a very long time. And arrows stuck in tree trunks or laying on the ground are deadsure marks of human activity. You don't want to announce your presence like that. Oh plus, I don't think you need to be notified not to use firearms. Instead, concentrate on fishing and foraging for edible roots, tubers and fruit. Oh plus, forget about growing any long term crops in your settlement. It's impossible without sunlight.

Rain

Yes, considering you are living underground, if rainwater collects in your little colony, it's going to get flooded really soon.

1- Try and build a sewerage system for rainwater. If there are any caverns underneath yours, that could come in handy. Build the sewerage system in a fashion that requires least effort to keep it running.

2- Try to not let the rainwater seep into your settlement. You can do that by tightly sealing all entrances that could let water pour into your colony. Of course you cannot watertight any entrances with medieval technology, but at least minimize the water inflow as much as you can.

Sanitary

In order to answer the call of nature, the citizens must go up to the surface. And after they are done, they must cover it up with a thick layer of mud. Do not let the stink announce your presence. If you limit going to surface for excretion to night times, the metabolism and biological clocks of your citizens would slowly adjust to it and they will not have any problems with it.

Time Of Activity

Your citizens are going to have to live the lives of owls and bats. Sleeping in the daytime and being active in the nights. Considering normal humans (and armies) are active in the daytime, reversing your time of activity would put the chances of being discovered, to a minimum.

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  • $\begingroup$ Unless the chamber pot hasn't been invented yet, the inhabitants will probably just use those, and only emptying the pots needs to be at limited intervals. Besides, unless they're chronically dehydrated, I'm pretty sure humans can't constantly go 12+ hours without urinating (especially pregnant women), never mind how babies are supposed to manage that schedule. $\endgroup$ – Matthew Jun 7 at 21:46
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Derinkuyu : Underground City

I don't have much time to write a lengthy post, but here is a Wikipedia link to an ancient, hidden, underground city.

Your question is actually a very close description to this real location.

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  • $\begingroup$ Thanks (Someone already pointed this city out with my previous question) but it does not solve the problem of hiding it from the outside. A "normal" city can't be hidden. An underground city is more difficult to find, but i feel like there are some tricks or precautions the inhabitants could take to ensure their security. They already have a fun place to hide, how can they make it perfect ? $\endgroup$ – Tyrabel Sep 18 '15 at 22:57

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