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I have a underground culture that has been cut off from the surface for near on 200 years, and I am trying to understand what issues will I need to cover and am most likely to run into for there survival to continue.

A little back story, the world becomes so over populated that they begin to dig into the depths of the earth, over generations they get so deep that some people end up spending their entire lives digging underground to create a better life for their family, millions live in the depths of the earth now and the government has called everyone to the surface to solve the world's over population problem once and for all, those who left never returned, but those who stayed began to fear going above the ground again since nobody returned, in the end they try to not speak of it between one another again and start to create a new life for themselves.

Points that might help get answers

construction would start around 2045

It's a large population of around 600k that gets halved to around 300k when cut off from the above world, I would like them to live at depths so far down that the idea of going back up is unthinkable or now to dangerous.

We have the old city's to check over for resources

Being cut off from the surface happens about 80 years after they start construction so we would have better technology to work with

A drug was administered world wide around 2125 which culls off 70% of the population only those that stayed deep underground are not effected

We have massive cities connected to one another so plenty of space after the drug was administered

Thank you all in advance

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    $\begingroup$ There is no level of overpopulation that requires us to dig underground. We’re only using a tiny tiny fraction of the land surface for buildings at present. Much of the rest is pretty uninhabitable, but it’s still easier to build and live in the Sahara or the Kalahari or Antarctica than it is underground. $\endgroup$
    – Mike Scott
    Dec 20, 2019 at 9:42
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    $\begingroup$ @MikeScott Certain parts of the world could become unlivable due to various reasons. Look at the current state of Australia and fast forward a few thousand years. This might reduce the livable space on the surface..but yea it is very unlikely that overpopulation would ever force us underground if things keep going like now. $\endgroup$
    – A.bakker
    Dec 20, 2019 at 10:14
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    $\begingroup$ @Mike Scott People have dug caves under cities in the past, to relieve local overcrowding (Nottingham, UK has some that were in use in medieval times). In those cases they still spent a lot of their time on the surface, but if advances in technology solved the food, air and power problems, then going further down would become a more attractive option. We already have hydroponics and geothermal electricity, it's largely a matter of making them cheaper and more reliable. $\endgroup$
    – Pastychomper thanks Monica
    Dec 20, 2019 at 10:41
  • $\begingroup$ Nottingham's caves were more about places for the poor to live than due to there being too many people for the surface. en.wikipedia.org/wiki/City_of_Caves $\endgroup$
    – puppetsock
    Dec 20, 2019 at 14:16
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    $\begingroup$ The theme of underground civilization cut off from surface is popular in fiction, however, overpopulation does not look a plausible cause. If overpopulation persists, the surface dwellers would inevitably reconnect with the underground. If overpopulation is reversed (and quickly!) there must be some calamity which caused it. $\endgroup$
    – Alexander
    Dec 20, 2019 at 19:50

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The main issues/Solutions are:

Air Supply: Without fresh air (Thank you trees) your population will die out. This could be remedied with air purifiers. (Remember the words Algae and Power)

Food supply: You need steady food supply, most animals won't survive extended periods of time underground, and when enclosed for several generations animals tend to evolutionary grow smaller so you are stuck with plant based food that need sunlight. You could use artificial sunlight which would require a lot of power to grow your food. One rather efficient food form would be algae which by some experts is already seen as the food of the future. Algae also has the benefit of efficiently producing clean air so this could help with the previous air problem.

Water supply : Water purifiers are already a thing but just like the previous two require power to maintain.

Power: Mainly because you need it for the above, fossil fuels are pretty much a no go, you could theoretically find these underground by digging but the problem is that the toxic by products have nowhere to go. "green" energies like solar and wind are a no go underground period. This leaves nuclear power the best option but then you do need to worry about radiation and the radioactive waste. For a population as small as yours the amount of waste created will be limited and you can dig far away enough to store it relatively safely.

Metals: Stuff breaks, you will need the material to replace what is broken, so depending on how deep and wide you can dig this will take you out on the long run.

Last issue is sanity: People are not made to live in enclosed spaces for extended periods of times, that's why astronauts and submarine crews are trained for it and not everybody makes the cut. A civilian population , children in particular, wild find this hard and probably lead to psychological problems. Also deficiencies in Vitamins mainly Vitamin D will become a problem.

Alternative things that might help:

cryogenic freezing and birth control to reduce the active population and save on resource consumption

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  • $\begingroup$ Firstly thank you so much for your answer I'm not sure if i can ask a second question in the comments section like this but If the power problem was solved do you think a Closed Loop Life Support system would solve the air issue, and would Geothermal energy be usable any way? $\endgroup$
    – Sandorien
    Dec 20, 2019 at 9:49
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    $\begingroup$ You can, not that familiar with "Closed Loop Life Support" though. But concerning any form of life support you will always be spending resources in one form or another because it is practically impossible to reach 100% efficiency. Geothermal energy could work but that would require digging deeper what get's harder/more dangerous. You will need an insane amount of prep time/Man power/resources to undergo such a venture $\endgroup$
    – A.bakker
    Dec 20, 2019 at 9:54
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    $\begingroup$ There is one more issue to warry about: heat; While geotremal power can and should be utilised in all other aspects heat management will be a great problem - there is no space to evaporate large amounts of water or just dump hot water. Heat from alomost any acitivity (including algae farms) would accumulate in place and make it unhabitatable. $\endgroup$
    – ksbes
    Dec 20, 2019 at 10:21
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    $\begingroup$ OP states that this starts 90 years into the future. So for energy you could state that they use a fusion power plant. Even more efficient than nuclear, no radioactive waste and the fuel (hydrogen) can be made from water. $\endgroup$
    – quarague
    Dec 20, 2019 at 12:50
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    $\begingroup$ @quarague Fusion will be 20 years in the future from then. It is always 20 years in the future. :) $\endgroup$
    – Michael Richardson
    Dec 20, 2019 at 23:04
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Any civilization needs a supply of energy to survive.

Being stuck underground means that the Sun cannot contribute to providing energy. This cuts off the possibility of feeding, since as of today we have no food source based on extremophilae, which are the only type of organisms known to live where the sun doesn't shine.

You might think that we can replace the Sun with artificial light sources. But also those need power, and where does that come from? If the underground city is built conveniently close to a mine, it can harvest coal, gas or uranium from there. But that leaves the demand of other minerals, like iron, copper and so on.

All in all, the entire scenario looks quite unrealistic: living close to one mine can be believable, living close to mines of coal, iron, copper seems a bit more of a far shot.

So, to answer your question, no, I don't think they can survive 200 years. They will run out of supplies way earlier than that.

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