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Underground societies have been a trope in fiction for a long time, but in the real world they don't seem to exist (at least not in the way they are often portrayed). Yeah, we sometimes see people living right near the surface in partially excavated cave dwellings, but from what I can tell large scale underground facilities remain limited to missile silos and generally groups really big budgets.

At first glance, it would seem like living underground would solve an interesting set of problems:

  • Heating and Cooling would no longer be necessary
  • Land could be used more efficiently (You can expand downwards, and leave the surface open)
  • Materials for construction could potentially be gathered during excavation

But it also creates a few new ones:

  • Massive amounts of material needs to be moved
  • Ventilation could become a potential problem
  • Radon and other radioactive materials pose a higher risk deeper underground

In general, people seem to gravitate towards doing things the easiest (and by extension usually cheapest) way they can, so I can only assume that large scale underground structures are simply not cost effective. So here's my question, what technology would need to be developed to make it cheaper than building above ground? Is it even possible, or is underground construction inherently less efficient?

TLDR; What feasible inventions could mankind develop that would allow them to live as mole people?

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    $\begingroup$ This doesn't answer the question, but all those problems would be overcome if the surface became uninhabitable for any reason. If you HAVE to live underground, you don't need any technology, you just need a pick and a shovel and a will to survive. $\endgroup$
    – kingledion
    Commented Mar 8, 2017 at 17:41
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    $\begingroup$ the biggest problem is always food, no sunlight means no base for a decent ecosystem that can support us. so they had better have really good hydroponics. $\endgroup$
    – John
    Commented Mar 8, 2017 at 18:26
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    $\begingroup$ Heating (and cooling, if you go deep enough) would still be necessary. You'd also need forced-air ventilation, lighting, and have more complicated water & sewage systems. You also can't just expand downwards indefinitely, as the remaining rock at tunnel level N has to be strong enough to support the N-1 levels above it. $\endgroup$
    – jamesqf
    Commented Mar 8, 2017 at 18:41

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There are no wonders. If you want to move large cities underground, you have to dig. A lot.

Radon concentration and similar things do exist, but - except some very few areas on the world - they are negligible. Although the highest purely natural radioactivity of the world is far higher as any prescribed health standard (around 0.8 Gy/year, living there means a much higher risk as smoking).

People doesn't really like to move underground, but if they have enough motivation, they will do. You don't need some cataclysmic event for that, it is enough if they can find work only there.

A radioactive surface isn't enough motivation, because in this case, also the ground water will contain a lot of solved radioactive materials, mainly metal salts, which doesn't make the situation better underground (while the increased costs of the digging still exist).

The main problem with digging, that it requires a lot of people, working a lot of dirty work, with big and costly machines. Consider the costs of the underground train tunnels. The Eurotunnel (binding UK and France) costed around \$21billion. The cost of similar underground tunnels costs mainly between \$10million-\$30million for every km.

Building the big and costly machines requires also a big and costly machine industry.

The better usability of the free, now empty surface land isn't a big advantage, because most of it is already uninhabited, even in largely overpopulated countries. We, humans, tend to concentrate ourself into big cities.

About the actual costs and technologies of the digging there is a quite useful SE site.

In any case, the opencast building (i.e. digging a big hole from the surface) is always much cheaper as digging essentially large mines.

Although heating won't be a high cost of these cave-cities, their cooling may be much bigger. And also their air ventillation. There are no wonders also in this case: you have to circulate water and air between the surface and (if it is irradiated, a closed water loop and air filters are adviced). You have to use pumps and ventilators for the task.

Note: there are radioactive gases, some of them won't be filtered by anything (noble gases). Although most of them decays very fast (thus they long decay as the caves will be built), or very slow (thus they aren't very radioactive). Fortunately, practically none of them is produced by normal fission processes (nuclear energy production + atomic bomb explosions), thus we have luck.

Fire, and any carbon dioxide producing objects (non-electronic cars) would mean much bigger risk and cost (air).

Energy production will still have a lot of contact with the surface, because there is no energy production without heat production, and you have to do something with the heat. Of course you can use surface solar panels and windmills to produce the energy on the land. In this case you will have a lot of cables.

The whole society will be much more strongly dependent from the technology, which will elevate its worth. The probable result will be that the law and the customs will follow the same. Smoking will be much more evil as now, destroying any technology, particularly if it has anything to do with the life-sustaining systems, will be much more hardily punished.

Working on technology (engineers, scientists, teachers of these) will be probably much more honored as today.

Sustaining the civilization will require much more work. It will have a negative effect to the life quality.

The civilization would require around 30-50 years to adapt (and, to reach the life quality of their old surface again).

If the move to underground is the result of a hard pressure, then the first years will be crucial and for the large masses, mainly catastrophic.

After such events - revolutions, downfall of a world system, world wars, etc - the society needs around 1-2 decades to stabilize.

I can advice two very interesting novels from societies in similar situations:

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So we already have the technology to do this. Humans have been developing the technology to exist underground since we first started mining.

The problem with your question is that, on an Earth-like world it will never be more efficient and therefore cheaper to build underground. You just can't get there, building down requires more work and will generally require you to fight gravity more often in the maintenance of said dwellings.

So the problems:

  • More expensive to build initially, moving material, digging, reinforcing etc.
  • Plumbing gets weird and will require extensive use of pumps
  • Air Quality, you'll have to have filters and you'll have to make sure air flow doesn't stop otherwise you end up with suffocation...you know dead people.

That being said there are reasons to build underground on a future earth or a planet only slightly different from Earth.

  • Storms. If a planet has regular, very strong storms, building up is a bad idea.
  • Lack of wood. Now its still probably cheaper to mine stone and build stone dwellings, or clay works...or sod. But its not unreasonable to think it'd be more common.
  • Creatures. If you have a man hunting beast it is way easier to secure an underground dwelling.
  • Post apocalypse...if the surface is a barren hellscape...

Inventions: Ironically we don't really need any new tech to make this feasible. As mentioned it is still going to be more difficult/expensive but with modern technology we can certainly create a stable effective underground dwelling.

  • Water systems. You are going to need pumps to REMOVE waste as opposed to needing it to get water, which is the flipside of how we do it today, but doesn't require anything special.
  • Ventilation. Special filters for radon and others stuff (as you mentioned) will be needed and the supply needs to be constant and need backups. This is particularly true with larger underground settlements. O2 is good for humans...
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Living and working underground is fine.

Huge expanses of empty limestone mines under Kansas City are used for all sorts of industrial and office space.

https://www.usnews.com/news/us/articles/2015/09/14/ap-news-guide-kansas-citys-network-of-underground-caves

But the coolest has to be the underground cities in Turkey.

http://www.goreme.com/kaymakli-underground-city.php

These are huge and very old. I am not sure how they addressed ventilation back before electricity. Especially if your only light was fire. One would think there would be giant piles of rubble on the surface nearby.

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  • $\begingroup$ Thanks for this info concerning Kansas City, very useful for my setting. $\endgroup$
    – Shadow1024
    Commented Mar 8, 2017 at 21:36
  • $\begingroup$ en.wikipedia.org/wiki/Coober_Pedy is another place. This is too get out of the heat. $\endgroup$
    – Catprog
    Commented Mar 8, 2017 at 22:54
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First of all, I don't think that people, given the choice, would like to live underground. Our ancestors might have lived in caves, but they spent most of their time outdoors.

As far as technology goes, the most important thing that we missing is good burrowing technique. Building tunnels is bedrock is very slow and expensive today. Erecting a building of the same volume on surface costs just a small fraction of underground building's costs.

Second thing is reinforcing the structure. When we have relatively small caves in a large amount of rock, the structure is stable. But if we want to use the most of the volume, walls and floors are becoming too thin to support it. Right now humans need to excavate the whole cavern and then build the walls and floors out of manufactured materials. This only makes underground buildings more expensive. If there was a quick and reliable way to reinforce rock, that would help. Another concern is earthquakes. For a surface building, we can design it with a certain seismic resistance. For underground tunnels, there is just no way of saying when they can collapse.

Other concerns, like ventilation, heating and cooling (yes we will need those too, even underground) and energy generation are relatively minor, but would also benefit from a more effective solution. For things like radon, carbon monoxide, carbon dioxide, methane - we just have to constantly monitor them.

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    $\begingroup$ Don't forget the very important need for true daylight. Consider that Suicide rates go up when you get far enough north for there to be 30 to 40 days of no sunlight. $\endgroup$
    – Paul TIKI
    Commented Mar 8, 2017 at 19:20
  • $\begingroup$ There are some people who build underground homes. I understand the preferred technique is usually to dig a big ditch, build the structure inside it (essentially above ground), then bury it. So yeah, building a whole city by excavation would not be cheap or easy. $\endgroup$
    – T.E.D.
    Commented Mar 8, 2017 at 19:27
  • $\begingroup$ ..also I understand underground homes (and basements) most persistent problem tends to be water leaks. Might add that to your list of issues. $\endgroup$
    – T.E.D.
    Commented Mar 8, 2017 at 19:28
  • $\begingroup$ I assume that underground cities would be built in bedrock, below the aquifer level, so water leaks should be a minor problem, if problem at all. Still, there is a valid problem with sewage flow and accumulation. $\endgroup$
    – Alexander
    Commented Mar 8, 2017 at 19:37
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It won't be about tech

As other answers already pointed out, we already have all the technology needed, and with any given tech level building and living on the surface will be cheaper. At least, technology-wise.

  1. Over population — if you have so much people that all your surface is either farming or last remnants of wild nature, then underground is cheapest place for living.

  2. Prices on terrain — rich people like to have land. There is not enough for everybody.

  3. Air pollution — and it's easier to keep it at bay in a cave than in dozens buildings with windows.

Et cetera. Just add factors that will make surface expensive, and you'll have underground economically viable.

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  • $\begingroup$ I believe that at present tech level, an arcology would be a cheaper solution than similarly sized underground structure. $\endgroup$
    – Alexander
    Commented Mar 10, 2017 at 17:21
  • $\begingroup$ @Alexander yes, and? I already pointed out that at any tech level surface buildings will be cheaper. That was the whole point of my answer. $\endgroup$
    – Mołot
    Commented Mar 10, 2017 at 17:42
  • $\begingroup$ I just can't see how land prices alone can force people to build 250-stories underground structure versus 200-stories above ground and 50-stories below ground arcology. $\endgroup$
    – Alexander
    Commented Mar 10, 2017 at 17:52
  • $\begingroup$ @Alexander in some countries, especially ones where mining have medieval tradition, ownership of surface land is separate from mining rights. So let's imagine that surface land alone cost thrice the cost of mining rights + underground arcology. Got it? $\endgroup$
    – Mołot
    Commented Mar 10, 2017 at 18:27
  • $\begingroup$ If we have this type of legal arrangement, then yes, that would work, assuming that underground arcology has minimal surface footprint and all excavated rock can be cheaply deposited somewhere. $\endgroup$
    – Alexander
    Commented Mar 10, 2017 at 18:39

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