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In the desert, the major factors of survival are water, food and temperature. I have a city in a desert where the first two are covered by a nearby river and agriculture from that river, but would it make sense to build a city underground to beat the heat? Or would it be better to adapt the species living in the city to higher temperatures?

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How far beneath the surface would the "floor" of your city then be? (If it has multiple levels, a range of depths is fine.) – type_outcast Jan 29 at 1:00
    
    
@type_outcast Deep enough to be insulated against the sun and weather/for the temperature to be livable. I don't know the exact number, though. – decayedarachnid Jan 29 at 1:25
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Coober Pedy is an excellent example of this actually happening. – Roland Heath Jan 29 at 2:02
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Sometime it does rain in or near deserts, and when it does there can be severe flash floods. So make sure your underground bunker is above the flood plain, even in the desert. – RBarryYoung Jan 29 at 16:21

Yes, underground would help with heat

Given food and water are covered, underground living will be cooler$^1$ and safer than living in direct sunlight, but there are some caveats:

Geothermal gradient

As you descend into the crust, the temperature increases in a steady, predictable fashion known as the geothermal gradient, which is about $25^{\circ}\text{C}/\text{km}$. Thus, your best bet is staying just a few meters below the surface. That way you'll get almost all of the insulating effects of the ground, while avoiding the increasing temperatures from below.

Digging is hard

Digging an underground city would be prohibitively difficult. Your people would be much better off to find an existing system of caves.

Ventilation and heat regulation

People generate a lot of heat and $\text{CO}_{2}$ that need to be exchanged for fresh, cool air. Putting multiple entry/exit points to your city will help, but you'll still need air flow, and this will certainly take a lot of effort!

You didn't specify the level of technology your people are at. Fortunately I can present a fan that requires practically no technology at all:

Put quite simply, you take a large piece of lightweight fabric, animal skin, whatever, and affix it to a wooden frame about $16 \times 16\text{"}$ ($40 \times 40 \text{cm}$). Make several of these. Then you have volunteers/workers/slaves at the (preferably ramped) entrances constantly push the air out. You will need one such fan for every 10–100 inhabitants depending on how densely packed the city is.

Inspiration: Nuclear War Survival Skills book

My inspiration came from the public domain book Nuclear War Survival Skills. Here is their diagram of the fan I described:

Simple box fan

At around p.59, they state that this fan can move 300 cubic feet/min, which is enough for 9 very crowded adults in hot weather, or up to 100 in cool weather.

That book describes some other fans and is in general a great read for designing underground living on a small scale.


With more technology, you can automate any type of fan somewhat by putting the fans on circular wheels or belts, and use some pulleys and gears to give the people (or beasts of burden) a mechanical advantage.

Again, ventilation is necessary (and very easy to underestimate!). It will be hard work, but if done adequately, the underground would remain cool and hospitable to human life.

Water source

You say you already have a nearby river. Does it run underground? Or is it at least somewhat near your underground city? If you can pipe some of it through your city, your inhabitants will have a much easier time with it, and the water will help cool the city even more.

Lighting

You didn't ask about lighting, so I'll keep it short: mirrors. Placed around your ventilation/entrance shafts, you can "beam" sunlight into the city. Your people would want to keep fires to a minimum, as they generate lots of carbon monoxide, which can be easily fatal.


  1. More moderate. Cooler during the daytime, and warmer during the night (a lot of deserts get quite cool during the night).
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Geothermal gradiant? At 25C/Km, that means you could go a fairly impressive 100m down and still only get +2.5C from that. – Dronz Jan 29 at 6:07
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Don't forget that water flow could easily be adapted to turn those fans too! – thanby Jan 29 at 15:04
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By building the tunnels right you can avoid manual fans. For example line an area with black rock in full sunlight so it heats up, the rising air there will pull air through your network. Fireplaces can also be used to achieve the same thing by pulling air in towards them. – Tim B Jan 29 at 15:33

Your question is bit misleading. The important thing is not to be underground, the important thing is to have thermal mass of stone around you. A historical example of what you want is Petra. Similar use goes up to stone age where people lived in caves that had stable temperatures year around. In addition to Petra other examples of rock-cut architecture exist in Cappadocia and India where it was a result of suitable rock formations.

In fact, I'd go so far as to say that such architecture will mainly exist in areas where you have natural caves or rock formations and easily workable stone so that the amount of work required is exceptionally low. Otherwise it will be easier to build above ground and just make the walls more massive. Typical solutions are adobe, mud brick, or packed earth which naturally allow relatively simple and easy construction of thick walls with high thermal mass.

Actually building underground would generally be impractical since while the resulting architecture would indeed have relatively stable temperature the amount of work required would be higher than with other alternatives with same protection from heat. Additionally being actually underground would make it more difficult to deal with floods and sand. It is generally better to live so that gravity helps you keep your home safe from such issues.

That said, it is reasonable for the desert city to have significant infrastructure underground. Underground aqueducts or qanats or likely. Similarly underground tunnels make a good source of cool air for ventilation. This would be combined with a windcatcher towers or similar.

I guess you could say that the optimum is a combination of below and above ground elements. And that building above ground usually requires less labor and is the default barring natural caverns or exceptionally easily workable stone or special needs as with water conduits.

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They could heap earth up around their houses and make them underground that way. They'd need strong walls and roofs. Do they have thick tree trunks? How are their roofs made now? Buildings made of stone with domed roofs could take the weight. – RedSonja Jan 29 at 8:52
    
+1 for mentioning Petra. I was going to do that :) – jwsc Jan 29 at 9:56
    
@RedSonja That is what packed earth is for. Added link to the wikipedia article about it to the answer. Although it uses "rammed earth". – Ville Niemi Jan 29 at 15:56
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I probably should expand on what @RedSonja said and my answer to it. The proper solution to providing thermal mass depends on the amount of thermal mass needed which depends on the magnitude of temperature variation, which is roughly proportional to the difference between maximum and minimum temperature and the cycle of the variation. While in a desert the temperature differences are large the cycle is generally between day and night. For that making the walls massive using methods I covered in the answer is enough, Earth sheltering is only needed for longer seasonal cycles. – Ville Niemi Jan 29 at 23:40

Yes

  • Insulation against sun, wind, rain, and temperature changes! The geothermal gradient can help keep a cave system warm. Some houses use geothermal heat pumps to warm or cool as needed, which can be extended to caves
  • Caves can often have natural choke points, allowing for easy defense.
  • Your city may be hard to spot. After all, it looks like any other bit of land!

The Other Issues

  • Light is an issue. Do you have skylights? Mirror systems? Do you use a lot of candles?
  • You need to dig out caves and rely on some structural engineering to keep things up. People have had great success, otherwise!
  • Fresh air needs to enter somehow. This requires ventilation, but that can be done.
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The fact that underground cities in the desert in our own world are very few and far between, as far as I know, might indicate that it would either require some special knowledge or else a whole lot of work to create an underground city in one. I think adapting to the heat or finding another way to cope with it would be easier, and more likely, personally, but then, I like heat, and I like to study heat-tolerant life. Nevertheless, I'm sure you could find a way to make an underground city in the desert (even a practical way), but it sounds like it might not be the easiest thing to figure out. An underground city might start to smell after a while, too (however, probably less in a desert than a humid area, is my guess). If it was close to a river, it might get a lot of water from the river running into it (and I imagine it would smell a lot more; the dry banks of rivers that once were wet can smell pretty interesting).

You might consider what termites do.* They have natural air conditioning with the way their mounds are set up. This would be helpful to establish a constant temperature (deserts get cold at night). In the case of termites, they keep it at about 30° C all the time, where it fluctuates between about 0–40° C. outside. They establish a constant draft of air, and build so that the sun doesn't hit it as much at certain times.

(*Note in the link that it says termites have a brain the size of a pinhead, but I've heard that science has established that the size of the brain doesn't regulate intelligence necessarily, but rather ability to control a larger body.)

You might also consider that the people of your desert might have materials that reflect infrared light (which could cool down whatever they're placed around a lot). Infrared isn't some special space aged thing. It's just a color you can't really see much, if at all (unless you've got super powers), because it's outside the visible spectrum for humans. (And, infrared light heats things up, much how UV rays give people sunburns, kill microbes, and stimulate vitamin D production. Plastic and modern glass usually block UV rays, and this helps to prevent damage of some kind or other—including to vitamins in milk and such, I believe. So, you would need a color that reflects infrared to keep things inside cooler.) In our modern world, you can get clear inserts to go on windows to block infrared (and stop the house from warming up through the windows). You could do the same thing to your whole house (or city) with the right color of paint or something.

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Have +1 for the termite idea. Was going to add it as a comment, but has already been covered. – Darren Bartrup-Cook Jan 29 at 10:14

Survivability is probably not an issue, as others have stated - increased insulation from the heat, etc. The difficult part of this is creating a history for such a city. One doesn't simply pick a spot in the desert and decide to build a city there, there has to be a back story to explain how it got there. One possibility would be for it to have started as a normal, above-ground settlement that over time became increasingly buried under centuries of sandstorms. People started building covered walkways between the buildings to keep out the sand, and building tall chimney-like structures, both to allow for air circulation and also some with ladders to provide a means of egress from the buried buildings. These chimneys would continue to be built taller as the sand became deeper.

Eventually, as modern technology became popular, they might also run electrical cables down some of the chimneys, as well as plumbing/sewage, and later telephone, ethernet, fiberoptic, etc. Some might even be converted into elevator shafts.

Prior to being hooked up to electricity, their primary source of light would be torches, which produce smoke, and so these would also need to be placed near the chimney structures. It's possible you could line the edges of chimneys with polished metal to act as mirrors and bring some daylight down into the buildings, but again, this means most of the light would be near the chimneys. Some enterprising architects might see the benefit of designing arched ceilings so that any smoke from light sources would collect into the chimneys.

One problem with this scenario is it does make it difficult to expand the city, as digging underground in sand is not a simple task. It's possible that additional structures would be built on the surface, which might themselves be buried by more sandstorms, so you have layer-upon-layer of city, with the deepest parts being the oldest and newer structures being closer to the surface. The most recent developments would be on the surface. Of course, any surface structures would have to be positioned such that they are not directly on top of the chimneys from the deeper chambers.

Eventually, social strata would start to form based on depth. The above-ground level would be mostly traders and craftsmen, the type who would do business with outsiders most frequently. Below them would be the aristocracy, who being the wealthiest residents would choose the most comfortable lodgings - deep enough to be protected from the heat, but not so deep as to have little access to sunlight, fresh air, quick trips to the surface, etc. After that, things would go steadily downward in terms of social standing, with the poorest people relegated to living in the oldest, deepest parts of the city.

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The best thing to do would be both. Snakes, lizards, and other creatures live in the ground during the day, when it's hot, then come out at night. You species could be nocturnal, resistant to heat, and be able to come out during the day if necessary. Being underground also helps insulate against all kinds of things.

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