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I was looking at these two questions:

Expanding Occupied Underground Habitations Safely

Does Living without access to Sunlight have known Physical/Psychological effects?

And in a story I'm writing, the characters are hurled into an impact winter progressing over a period of weeks. And so, in an attempt to survive where help isn't coming, they search for other survivors and so on. And so I'm wondering, when these survivors get together, assuming an asteroid has hit elsewhere and screwed up the ozone/blocked the sun (partially? All of it? Not sure how that works.) - would an ideal place to re-start civilization be underground? What challenges would be faced and what would the benefits be?

Also, bonus points if you think parts of the NYC subway system that haven't been flooded/destroyed yet could be used as a starting point for underground "cities".

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  • $\begingroup$ An impact winter is mostly dust in the upper atmosphere. Essentially the sky would be permanently overcast, you just don't see the sun for years at a time. $\endgroup$ – Tim B Oct 15 '14 at 20:08
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    $\begingroup$ The main reason it kills isn't the winter itself, it's the fact that all the plant life can't grow - imagine a winter lasting 5 years. Anyone who doesn't starve to death during that time can rebuild fine after though. $\endgroup$ – Tim B Oct 15 '14 at 20:58
  • $\begingroup$ @TimB So would there be no alternate means of growing plantlife if it's say, in their underground home? With the winter ongoing and the sun overcast, is there no chance for plants? $\endgroup$ – KaguraRap Oct 16 '14 at 2:36
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    $\begingroup$ @TimB - If you believe Game of Thrones, the best way to survive that is to have lots of nudity, betrayal and Civil Wars. $\endgroup$ – Oldcat Dec 17 '14 at 19:44
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Underground shelters could help improve survivability in the first days/weeks after an impact, as it provides shelter from raining debris and can also allow survivors to breath cleaner air that has fewer particulates. For the most part, though, this only affects those relatively close to the impact itself (but, of course, far enough away to survive the impact).

After the initial impact and the immediate effects have passed, however, I wouldn't call it ideal. While there are certainly advantages to living underground, arguably the biggest challenge of rebuilding civilization there lies in expansion; on the surface, you can expand to your heart's content (barring natural barriers that, for the most part, can be worked around, or just expand in a different direction instead), but underground you have to devote considerable manpower, energy, and resources just to give yourself the space to then build within. Granted, digging techniques could be adapted such that much of the "build within" is merged into the "dig the space" -- e.g. dig a tunnel, then a doorway in the side and a home on the other side -- but you're still devoting considerably more resources to the effort of expansion (it's a lot easier to build a house than to excavate one!) that you cannot then devote to other considerations of growth, such as maintaining utilities, working farmland (I'm assuming you've got appropriate methods for underground living here), developing infrastructure, re-discovering important technologies, etc.

One thing to keep in mind in an Impact Winter scenario is that it's nowhere near as apocalyptic as Hollywood has portrayed it, and despite the colder temperatures (which will warm back up as time goes on) the surface is nowhere near hostile enough to require survivors to bury themselves in underground cities.

On the other hand, you could easily have a group of survivors that have seen one too many Hollywood apocalypse movies who do assume their best odds of survival are underground -- if life has taught us anything, it's that just because they're wrong doesn't mean that people won't still do something!

Other points:

Advantages:

  • Easier to heat/cool to comfortable temperatures
  • Increased security (fewer access points)
  • Shelter from debris and particulates (quickly moot within days/weeks of the initial impact)

Disadvantages:

  • Food: You'll be unable to rely on natural growth and will have to devote energy into artificial means such as hydroponics with electric grow lights
  • Growth: You'll be very limited in how much space you have, and very restricted in how quickly you can expand said space
  • Ventilation: Without artificial (read: electric) methods of improving ventilation, air can quickly grow toxic, and any underground gas pockets that are accidentally breached can be quickly fatal to the entire community
  • Confinement: Should something happen and you have to evacuate (e.g. gas pocket), you've got fewer egress points and will quickly find panicking people trampling one another far more than in a surface community
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  • $\begingroup$ It's not apocalyptic? T^T I've seen articles where Impact Winter that happens after an asteroid can wipe out life (e.g.dinosaurs). Also this game is based on Impact Winter as well: kickstarter.com/projects/mojobones/impact-winter $\endgroup$ – KaguraRap Oct 15 '14 at 20:10
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    $\begingroup$ @KaguraRap If that's the impression I gave, that's my mistake. I mean it's not as apocalyptic as Hollywood makes it out to be. No absolute-zero temperatures anywhere, and no crazy vortexes sucking down super-cold air from the upper atmosphere. It's not going to be pleasant, that's for sure, but the surface isn't going to be anywhere near inhospitable, especially for humanity which is more than capable of adapting to colder weather -- heck, you guys down south will still be warmer after the impact than we are up here right now! $\endgroup$ – Kromey Oct 15 '14 at 20:24
  • $\begingroup$ lol I looked at your profile - Alaska? I'm cold just hearing it. Well I understand Impact Winter may kill off a lot of people caught off guard, but the main damage that would kill them off would be from lack of plants/animals and limits to travel due to temperatures and snow, pipers bursting, poorly built buildings collapsing and so on. But it can be apocalyptic then. Just no "Day after tomorrow", got it haha! Thanks for clarifying. $\endgroup$ – KaguraRap Oct 15 '14 at 20:30
  • $\begingroup$ Thanks for the breakdown @Kromey. So basically underground for large civilizations are no good for long term and even during Impact Winter, would be unnecessary? Is there a chance that if they remain in their homes above ground while the temperature reaches ridiculously cold, that buildings/cities can become stop being functional? e.g. bursting pipes, collapsing buildings, no transportation, stranded and desperate survivors/looters, and so on? If Impact Winter caused a continuous snow fall for a period of years? $\endgroup$ – KaguraRap Oct 16 '14 at 16:50
  • $\begingroup$ @KaguraRap On second thought, that's just beyond the scope of this question -- though if you pose that as a new question I'll be happy to post an answer to it there. The short, comment-worthy answer is that yes, buildings and infrastructure will have serious problems, but if you want to know more you'll need to pose that as a new question! ;) $\endgroup$ – Kromey Oct 16 '14 at 17:06
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Their main problem would probably be to find food. Plants don't grow underground/without sunlight. Of course, if they have sufficient supply of canned food to survive until the surface is habitable again, that would solve the problem.

However without fresh food, there may still be deficiency diseases, especially Scurvy. However again, this problem may be solved with supplies of food supplements or specific long-lasting foods like pickled cabbage. Also maybe it's possible to temporarily come to the surface (potentially with appropriate protection measures) to collect fresh plants even if living on the surface is otherwise not possible.

The problems of missing sunlight have already been explored in the other question you linked to.

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    $\begingroup$ Mushrooms could grow without light, if it were possible to keep them substrated $\endgroup$ – sum1stolemyname Oct 15 '14 at 20:01
  • $\begingroup$ @sum1stolemyname: I doubt that you could survive for a longer time only on mushrooms. $\endgroup$ – celtschk Oct 15 '14 at 20:02
  • $\begingroup$ @celtschk Upvote for the mention of deficiency diseases - I hadn't thought of that. Thanks. :-) $\endgroup$ – KaguraRap Oct 16 '14 at 16:51
  • $\begingroup$ Realistically there will be little if any food to find--the land will be stripped bare of anything worth eating by the crowds of survivors. $\endgroup$ – Loren Pechtel Jun 26 '17 at 13:35
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How big was the asteroid? Why are there only a few survivors left? If it was because the impact itself killed most people off, the asteroid was very large and has created significant problems well beyond what would be faced if it was a small asteroid which triggered human-caused mass death (religious extremism bringing worldwide violence, fear causing nations to desperately grab for resources causing WW3, etc).

The larger the asteroid, the more extreme the effects. Hot debris could cause global firestorms, resulting in a low-oxygen high-CO2 atmosphere - a couple of years of darkness and serious acid rain killing whatever survived the firestorm and killing off aquatic life too, followed by a hot earth as tiny life regrows. Big enough to wipe the ozone is big enough humans won't survive.

It will be difficult to thread the needle on big enough to kill most people directly, but small enough for scattered survivors to make it (presuming there is even any area in-between - I suspect big enough to cause eventual extinction comes before big enough to kill most people on impact).

If a smaller asteroid, the impact is more of a narrow regional problem with the exception of reduced sunlight. This isn't going to result in glaciers retaking to world, just relatively normal wintry temperatures holding year round (spring comes in a year or two instead of a couple months). Massive food shortages (no natural growing season for a year or 3) will cause global disruption and worldwide starvation, but some will be grown in greenhouses with supplementary grow-lights, probably in addition to aquaponic systems if you can get them set up (I foresee diets of bugs and farmed algae).

If you want scattered survivors after an impact, your best option is probably to go with a small asteroid, whose global effects are really just limited to not being able to grow food for a year or two, followed by human-caused disaster. Religious revivals (it was the angry fist of god) leading to mass warfare as the fanatics purge the unbelievers, plus skittish governments starting panicked wars of desperation to control resources, and general social panic being the source of collapse leaves civilization collapsed but humans not extinct. Nuclear exchange in the aftermath of impact would give you fallout as a reason to stay underground, as well as defensible shelter from roving bands of the various religious inquisitions.

People need sunlight and that is going to be difficult enough from the ash clouds - every moment of daylight should be spent outside, and even then they should be keeping a power plant going for greenhouse grow lights and supplementary lighting for vitamin D generation (especially to keep children from getting rickets). There better be a very good reason to stay indoors, and no asteroid is going to give you that without also spelling extinction.

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In your scenario 5 movies come to mind (A kid and his dog (people living underground after a N-war) Logans Run (Life during a Ice Age), and another three where a Asteroid hit. 1st one is a Big Ice Age after and the other is it hits Minneapolis - sort of a religious movie, last one is about a scientist who preps with the people who believe him - all else croak.

Back in the 70's it wouldn't be possible - or at least back to primitive living. Now in 2017 in MN we have a Hydroponic factory that supplies us with Tomatoes year round (they can not compete during the peak in summer) and several aqua culture places that raise fish (tilapia) and plants (lettuce and such). With the Solar Minimum coming we should be able to survive if the snow doesn't get too deep and the Power Plants keep on. In Turkey there's a place that has a underground city from several hundred years ago. So it was possible back then to survive.

One thing about now also is there is a lot of people who are "Preppers" and worried about things happening. So your book would be good if you write it for the Prepper people. There's been movies already (probably based on a book or so) that I outline above.

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