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I'm designing a Fantasy AGE campaign and there is one place in the desert between two big kingdoms. There is a way through the desert passing though several cities. They are mostly small and serve as trading outposts, but I want them to be bigger than that: to have tunnels ways through the sand and bigger cities under the sand itself, where most people can't get.

How deep should they be, what problems can characters meet because of the tunnels and undersand cities (like big hollows under the sand? I don't know, that's why I'm posting this), can that actually exist, or should I just say "FRIKKEN MAGIC!" and don't even bother?

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  • $\begingroup$ So, we're talking about a sandy desert, right? Not just a dirt-covered plain? Because if you have little to no sand, you can have as many tunnels as you want. $\endgroup$ Sep 16 '15 at 19:27
  • $\begingroup$ Most people can't access the larger cities below. They are uninhabited? $\endgroup$
    – Vincent
    Sep 16 '15 at 19:34
  • $\begingroup$ @DaaaahWhoosh yes, it is meant to be a sandy desert. $\endgroup$
    – Arbaks
    Sep 16 '15 at 19:45
  • $\begingroup$ @Vincent nope. I meant to say, that they are habited and alive, but not accessible by outlanders, except for some special people, who earned trust of the undersand rulers. $\endgroup$
    – Arbaks
    Sep 16 '15 at 19:50
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    $\begingroup$ Consider looking into the Fremen from Frank Herbert's Dune. Their Sietches are smaller than you probably want, but they point out a key thing about the desert: if you don't know where you're looking, its really hard to find something! A city might be a mere 5m under your feet, and you'd never know it's worth digging there! $\endgroup$
    – Cort Ammon
    Sep 16 '15 at 23:22
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If I'm picturing what you want correctly, here's what I would recommend. The source for the underground bit is from when I was an urban planner in the middle east.

Distance

Your settlements should be no further than a two or three days trek by camel/caravan. While you can go further, some of your perishables (dates, fish from the sea, camels milk) should be kept in a cool place, and by day, the further from the sea or your two mega-cities will get very hot even in the winter. You ideally want respite and water for camels as well after a couple days. Wayfinding for settlements much further apart gets difficult, if you're off by even 1°.

Depth

Water and sandstone can be found in just 3m depth in the Rub Al Khali. Sandstone is marvelously pliable (see this question). There are also many instances of troglodytes in desert areas, particularly the many in southern Tunisia and North Africa. For instance, Matmata has just a few entrances, but holds a whole warren of tunnels and dwelling units.

Size

Your size of underground caves is up to you. However, keep in mind that you need access for materials in and out, including waste. The old oasis of Tozeur was about 700m x 700m, but you want yours to be underground of course. This is because the 5min walking distance from the central mosque is considered about 350m radius in planning terms. Presumably your central entry/exit could be the same - give your underground dwellers a 5min walking distance from the entry/exit.

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This can happen. Because it has happened, thousands of years ago.

Take a look at the Derinkuyu underground city and its brethren. Room for 20,000 people, five levels, kilometer-long tunnels, &c. (Not much of a view, though.)

Here's a schematic from the linked article. (Note that the town above has grown up above the original cave systems.)

tourist schematic of Derinkuyu

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Problems to overcome

  1. The first problem I find when building large cities in the middle of a desert is water. If you want a lot of people, then you need a lot of water. Actually, the most important cities, regardless of climate or ecology, were almost always built near a ready source of water; but to drive that point home, you can take a look at a map of ancient Egypt.

    At some point, your city cannot grow past its usage of available water. So if you want large, sprawling cities, you need a lot of water, and if you have a lot of water, then you don't really have a desert.

    So, let's say there are giant underground water reserves. That obviates the need for surface water, but we still have to get to it. Which leads me to...

  2. The second problem you'd have to overcome if you want your undersand cities is actually just digging in the sand. Take a shovel to the middle of a sandy dune and start digging...what happens? The scoop of sand you just dug out just fills back in with more sand. Of course, at some point, you'd hit bedrock, but not after a feat of civil engineering. So, you have to keep your workers alive for the time that they are working in the sand. In a hot, dry desert. The outcome of this project had better justify the crazy expenses that you are incurring. Your best bet is if the desert already has rocky hills/outcroppings...then you can tunnel in that way.

  3. Other than that, you have some just general considerations to think of if you want to build underground cities. How are you going to get air? Light? Food? All of these things are harder to get underground, which is why most cities are built above ground. Typically, humans tend to fill their needs by using the path of least resistance. Underground cities present a lot of resistance, so there had better be a big payoff to living underground. Living in the desert also provides a lot of resistance, which is why you don't see a lot of classical cities that sprang up in the middle of a desert.

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  • $\begingroup$ +1, but I think the OP only wanted tiny settlements in the desert between the two large cities; in which case groundwater (such as found in oases) could be sufficient; and digging in the desert is not so tough - the compacted sandstone can sometimes be found just inches below the surface sand. But agreed, overall it'll be tough. $\endgroup$
    – Mikey
    Sep 16 '15 at 20:24

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