Background: The 20-km (or however large it needs to be to cause the below effects) asteroid appeared out of a dimensional rift close enough and going fast enough that it will reach Earth in just six or so months. It's projected to land in the middle of the Pacific Ocean, right in the ocean crust near Hawaii.

Technology Levels: Present Day.

Impact Risk and Aftermath: Astronomers have been unable to identify the composition of the asteroid, and they want want to err on the side of caution and assume that it's dangerous. They recommend earthquake-resistant shelters closed off for the outside world to reliably survive. Electricity, air, food supply, and water is necessary for the duration of the projected 5-10 year wait.

Following the impact, an earthquake rips through Earth, collapsing any shoddily built shelters. The usual aftereffects of volcanic activity, firestorms, and dust clouds lowering global temperatures. Whereas you could usually go outside soon after everything settles, the "unknown substance" prevents that.

The Question: If almost every country that's currently not fighting a civil war decide to start building or upgrading self-sufficient underground shelters, about how many people can they save? How many can each country save?

Bonus/Alternative Questions: If technology today isn't enough to build a large-scale shelter, how many years in the future is reasonable for governments to choose this option? How big would such "community/peasant shelters" be?

Edit: Underground and underwater is NOT necessary.

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    $\begingroup$ Just FYI. There are many asteroids we don't know about until they are much closer. Dimensional rift isn't needed. It can be a dark asteroid coming from deep space that is spotted by chance. Also welcome to the site! Please don't hesitate to take the tour and check the help center. $\endgroup$
    – Trioxidane
    Commented Jun 11, 2021 at 8:08
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    $\begingroup$ Astronomers have the instruments and experience to pretty readily classify asteroids by type/composition and obtain generally good estimates of their mass and composition. Mass is important BTW. A 20 kilometer wide asteroid composed largely of nickel/iron will do much more damage than one composed of porous rock or ice and frozen gases etc. $\endgroup$
    – Mon
    Commented Jun 11, 2021 at 8:20
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    $\begingroup$ It is very much easier to build an earthquake resistant, sealed to the outside world facility above ground than underground. Since the story insists that they want to build their shelters underground, this means that in season 1 there was a worldwide epidemic of insanity, rendering all people in power terminally stupid. Since the effects of this devastating mental disease are unknown, there is no way to know how much food and water they can stockpile in six months. $\endgroup$
    – AlexP
    Commented Jun 11, 2021 at 13:55
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    $\begingroup$ Does the general public know (mass panic, societal breakdown)? Or just a few scientists (organized selection of the few, better access to resources, it's all done quietly with no panic)? I'd suspect it might make a big difference to the outcome in terms of numbers. $\endgroup$ Commented Jun 11, 2021 at 14:41
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    $\begingroup$ The general public knows because they have telescopes. Something that big can't be kept secret. I decided that underground isn't mandatory! Also, the dimensional rift is just a story thing. $\endgroup$
    – Henry Shao
    Commented Jun 11, 2021 at 17:36

10 Answers 10


Food is a (the?) major limitation

In six months there needs to be 5-10 years worth of long-life food put aside. Let's take the low figure of 5 years as a less impossible target. Assuming that there is normally enough food production in 6 months to feed the population for 6 months, now the production of food must be increased tenfold for the next six months, with 90% of that food being suitable for long term storage.

This is not possible - Earth cannot instantly ramp up its food production tenfold. What will happen is that the cost of food will instantly skyrocket as wealthy would-be shelter builders try to hoard all available food. As soon as the cost of food increases beyond the ability of people to feed themselves today, there will be riots. As soon as countries cannot feed their populations, there will be wars.

(The early days of COVID showed how vulnerable supply chains are, even when governments are doing their best to maintain trade with a fair degree of cooperation. In a zero-sum game with food scarcity, world trade would disintegrate instantly.)

At this point it becomes unlikely that any shelters will be built, simply because the widespread ongoing conflicts will cut the supply chains for not only the food but all the other materials required for building ultra-strong shelters with reliable, long duration closed system life support and power. A decisive, ruthless military group able to seize control of a large food production area may be able to get a shelter built and stocked, but only if they can convince all of the key people that they will get a spot in the shelter.

My best estimate is that less than 1% of people in major food producing areas of developed countries would end up in shelters that have a chance to last 5 years. More to the point, upwards of 90% of the world's population would die from starvation or warfare in the 6 months prior to the asteroid impact. This is an easily foreseeable outcome for the world's governments. Given this, a rational government would need to be absolutely certain that the asteroid would render the surface uninhabitable for 5-10 years before taking actions that would definitely kill the majority of the world's population.

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    $\begingroup$ I think food production after the impact will be the even bigger problem. If the surface is not safe for humans, how can it be safe for farms? But yes, no ways we accumulate supplies for 5 years, for everyone on short notice. Not without stealing from those that will be left on the surface $\endgroup$
    – PcMan
    Commented Jun 11, 2021 at 13:39
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    $\begingroup$ Are things like underground facilities for farming buildable in that time? $\endgroup$
    – Henry Shao
    Commented Jun 11, 2021 at 17:51
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    $\begingroup$ @HenryShao, to put it into perspective, the Chunnel had 11 tunnel boring machine running for around 2 years to make 2 tunnels 25 ft (7.6 m) wide and 31 miles (50.5 km) long. en.wikipedia.org/wiki/Channel_Tunnel#Tunnelling That's insignificant as far as crop land or animal pasture goes. That's only about 95 acres (per tunnel). You need at least 5 acres per person to survive, so that's 38 people total, maybe. primalsurvivor.net/much-land-need-self-sufficient You can increase that with hydroponics, but not to the extent where one tunnel would feed millions, unfortunately. $\endgroup$ Commented Jun 11, 2021 at 18:37
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    $\begingroup$ @user3153372, the problem with simply growing food in more locations is that it takes a long time for many crops to become ripe enough to eat. For example, corn can take 90-120 days just to sprout. farmprogress.com/story-once-corn-planted-how-long-take-9-112050 Shorter growth times can help, with lettuce being 42-52 days to sprout, but we can only eat so much lettuce. gilmour.com/growing-lettuce We'd need more, but that's still a very limited diet. growveg.com/guides/… $\endgroup$ Commented Jun 11, 2021 at 18:42
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    $\begingroup$ @HenryShao re: underground farming facilities in 6 months, probably yes, if you're willing to give up eating meat (at least with today's technologies anyway). Aquaponics with grub growers is a closed loop farming system often used in doomsday-esque shelters with the downside of limiting your diet to fish and greens. Main issue you'd encounter is running out of fish in such a high demand scenario. $\endgroup$ Commented Jun 11, 2021 at 20:34


You've got requirements for well built and self-sustaining, it takes around a year to build a decent house. Self sustaining is complex and that alone will probably take a year to set up requiring large amounts of space per person to be supported. Chances are that 6 months isn't enough time for a government to get a plan off the paper and as far as breaking ground for the first bunker.

What you could do in six months is build some shoddy bunkers out of existing metro stations or old existing bunkers, stuff them full of preserved food, add a crude closed loop water and air supplies and a nation could possibly manage to save a few hundred people. Though that doesn't consider how you're going to power the place.

Of course the official government nuclear bunkers will still exist as will private bunkers, and the people with access to those may survive. Though it raises the question of how well stocked they are.

  • $\begingroup$ I think you mean an immaterial amount. I think a lot of the bureaucracy and waste of the government will disappear in an apocalyptic scenario. If nothing else you have millions of volunteer human laborers. At least in the areas where manpower can be organized effectively, there will be survivors. Even if they don't make the 'well built' criteria and nothing went exactly as planned. $\endgroup$
    – TCooper
    Commented Jun 11, 2021 at 23:45
  • $\begingroup$ @TCooper the difference is statistically insignificant, not enough for a sustainable population in the aftermath, ultimately everyone dies. $\endgroup$
    – Separatrix
    Commented Jun 12, 2021 at 7:09
  • $\begingroup$ nbcnews.com/mach/science/… I disagree whole heartedly that there wouldn't be a sustainable population after. Civilization as we know it is forever erased - but I doubt the human species dies out entirely. $\endgroup$
    – TCooper
    Commented Jun 14, 2021 at 17:53
  • $\begingroup$ @TCooper that's an unusually low estimate for minimum viable, especially when we have examples of ethnic groups with very small population bases and the troubles they already have after only a couple of centuries $\endgroup$
    – Separatrix
    Commented Jun 14, 2021 at 18:01
  • $\begingroup$ Well hopefully people won't be worried about maintaining some odd ethnic purity in a post-apocalyptic world and would "interbreed" freely. I'll admit there are still backwards people in the world, but seems to be a moot point in 2021. I think 98 is low, as its ideal, and probably requires a specifically diverse genetic population to be viable at such a low number, but I'd venture most major world powers government + private bunkers would host 98... maybe I'm just too optimistic, but I'll keep choosing to believe you're too cynical, hopefully we never find out who's right $\endgroup$
    – TCooper
    Commented Jun 14, 2021 at 18:13


@Separatrix already mentioned that you will probably not be able to save anyone, but I can give some more perspective from paleontological history when this happened before (read, the K-Pg/K-T Extinction that wiped out the dinosaurs). The situation is even more grave than most people here are giving credit for. Even if you are able to get these bunkers constructed, humanity will not be able to survive the meteoric apocalypse. There is just too much riding against them.

I gave some of this information in a similar answers I gave to other questions about human technological collapse and the end-Cretaceous extinction, so if I forget anything here checking that might help.

One thing that isn't greatly appreciated about the K-T extinction is that it isn't just the dinosaurs and big reptiles that went extinct, but that most of the little stuff died out as well. Most birds went extinct, including the Cretaceous equivalents of songbirds, and even larger mammals and turtles like Didelphodon and Nanhsiungchelys, which were the Cretaceous equivalents of raccoons and tortoises, were wiped out. So that suggests whatever happened at the K-T even small, adaptable animals will die. Humans will die, no ifs, ands, or buts. We're just too big and require too many resources to survive in such a scenario.

After the K-T impact, photosynthesis basically stopped for at least two years and perhaps as long as a century (two years seems to be the better supported amount), and the climate was so screwed up that farming wouldn't be practical for 20 years or more. The issue with this is depending how long such a nuclear winter event lasts, your human shelters may end up using up all their food and be forced to scrounge on whatever they can find outside of their shelters. Even after the impact is over growth may be negatively impacted, most of the plant life that returned immediately after the K-T extinction for the first 100,000 years or so was ferns, which are tricky to eat because often only the fiddleheads are edible and only for a limited period of time, otherwise ferns can be very poisonous. The soil may not even be super fertile in the after-event of the K-T, so you may not be able to grow crops. This turn of events would be occurring for much longer than any shelter could reasonably stockpile canned goods.

Storing fuel is also going to be a huge issue. Nuclear and fossil fuels probably can't be stored in great enough amounts for people to maintain long-term stability after the event. Maybe enough to get through, but they'll eventually run out and will not have the infrastructure to get more. Hydropower and wind power will be devastated by the impact. And solar just plain won't work for the first few years due to the rays of the sun being blocked off. Geothermal is the only thing that would work, but that would require your shelters being placed in very specific areas that would make them vulnerable to post-impact looters.

Also note that we still haven't managed to create a long-term sustainable biosphere, much less one that also supports humans. Biosphere 2 was a disaster that only managed to survive for two years at maximum (they tried the experiment twice) before being shut down, in both cases the experiment failed due to running out of food and oxygen as well as human nature ruining everything as it usually does. That was a 3.14 acre enclosed space that failed to sustain eight humans. We have no hope of riding out any mass extinction scenario in bunkers, especially at our current level of technology.

Best case scenario is you build your geothermal shelters, run through your supply of food and are then forced to search outside for new sources of food, only to starve long-term due to a lack of vitamins, minerals, fats, and proteins in your diet from the only thing being around in sufficient numbers to eat is fern fiddleheads. More likely, the bunkers get broken into and torn apart by desperate citizens trying to find a spot to ride out the apocalypse. Large amounts of people are not going to just lay down and die with grace if they know there is a chance for survival elsewhere.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Commented Jun 15, 2021 at 19:27

It depends on where you are - and unfortunately not everyone.

During the days of the cold war, underground bunkers were quickly constructed in the event of a nuclear exchange. These had all the features you were looking for.

It is important to note that it was thought nuclear war was imminent shortly after WW2, so the US Government (and indeed many others) created comprehensive plans to safeguard as much of the population as possible, often using a variety of ways to do so.

The best way to achieve this was to have a multi-pronged strategy in such a short time. So don't 'put all your eggs in one basket', instead do many things at once:

  1. Governments create purpose-built large shelters from scratch:
  • All your world governments would quickly utilise all economic, labour and military resources to building these.
  • Acquiring mining equipment and quickly compulsorily possessing ideal land would occur
  • I would imagine using all government resources a fully functional, self-sufficient shelter could be built within 3-4months as was the case in the 1960's, and many of these would be built on the outskirts of the capital cities.
  1. Re-purpose existing building basements to easily convert many into shelters
  • This was popular during the cold war - as a lot of the infrastructure and building structure makes this easier
  • Many large buildings have large existing basements that could be earthquake resistant, and with modifications can be good shelter candidates. I would imagine these could be done 2-3 months with adequate resourcing.
  1. Tell the populace to 'build their own'.
  • The US Government gave many instructions and guidelines at the time to get the public to create their shelters privately.
  • By doing this, the US Government at the time saved a lot of effort and resources, with many being able to build shelters on their own properties themselves or within the private sector.
  • However in your case, there would suddenly be great demand for products and food required to stock these shelters - so the government would need to be strict with rationing and ensure product and food supply chains are kept intact.

Even with the above though, there would be those that would miss out. I would imagine:

  • Governments without access to many building, product and food resources would struggle. Most in Europe, US, Australia would be able to accomplish mostly using the above techniques, but island nations, remote countries, landlocked small countries would require large amounts of aid.
  • Countries like those in Africa and Asia that have high populations but low per-capita capability would similarly struggle. India, central african countries, many in South East Asia, would likely find it difficult to obtain resources and products needed.

Unfortunately, most of the population of the world exists in these disadvantaged areas, and using predicted death rates from nuclear war in cities in these areas as a guide I would imagine that perhaps not more than 25% of these populations may survive.

However in the US, Australia and Europe you may expect perhaps 50% of the population would be able to be saved if their Governments acted quickly and with impunity. (In fact, Switzerland has 114% shelters already, Sweden 70% without any action - so these could save all their citizens already).

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    $\begingroup$ Just adding, if you're near the Pacific Ocean, this thing likely will create a tsunami worth avoiding (fallout shelter or not). $\endgroup$
    – Mikey
    Commented Jun 11, 2021 at 19:25
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    $\begingroup$ Well, most tsunamis are worth avoiding. $\endgroup$ Commented Jun 11, 2021 at 20:24
  • $\begingroup$ It wouldn't matter much where you are. A 20+ km rock slamming into the pacific ocean, might as well be hitting land. It will instantly vaporize any water in it's path and slam into the ocean floor, firing molten rock into earth orbit and beyond. Most of that material will rain back down onto the planet, raising the above ground temperatures to hundreds of degrees. Then when things start to cool down a day or two later, it will start to rain. A torrent of water lasting months and flooding every inch of the earth. $\endgroup$
    – jwdonahue
    Commented Jun 12, 2021 at 0:56
  • $\begingroup$ If more than 1% of the population survived such an event, it would be a miracle. Most of the shelters would be retrofitted mines, not fresh digs. Many would be at least partially destroyed within a few weeks of the impact, by repeated earth quakes. There would be many quakes in the following weeks on the order of 8..12 on the Richter scale. These would decline in magnitude over the first few years, but we'd experience after-shocks for at least a hundred years. People will survive, but it's likely to be numbers in the hundreds to maybe a few thousand. $\endgroup$
    – jwdonahue
    Commented Jun 12, 2021 at 2:21
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    $\begingroup$ I would also point out that most of what the world's governments did, regarding nukes, was simply to alleviate the publics concerns, and would not have been very effective if the worlds super-powers had gone to all-out nuclear war, which they were planning to do if attacked. They did build some really nice bunkers for key military assets, politicians and their aids and families. And that's about all they could do in with six months of warning. Fortify some existing bunkers, and retrofit some mines. Just about everybody would die anyway. $\endgroup$
    – jwdonahue
    Commented Jun 12, 2021 at 2:28

Impact location is known, so make sure you evacuate.

Because of the meteor showers following the impact, it will be a mass extinction event for anyone or anything outside on the surface. Shelters could help to bridge that first danger. But not everywhere. People will need to evacuate in time, from any coastal regions. An underwater shelter is a place you'll drown, instead of survive.

Agree strongly with "depends where you are" aspect.. I'd like to consider geographic differences.

If you'd have world wide seismic damage from an asteroid, it will be large enough to evaporate a large part of the Pacific ocean and form lava seas. one world half - the pacific hemisphere - will be affected far more severely. East Asia, Australia, New Zealand and America will loose their coastal cities, the area will be devastated.

In the pacific region, bunkers won't help, even Kim's shelter won't be heavy enough to sustain the shock wave. On the other end of the planet, Europe, Africa, western part of Asia, meteroid showers will cause massive damage, but shelters can be made and many would survive the impact. The pacific coast will not be repairable: humanity will loose a lot of good land..

Take into account shelters will not prevent all deaths after the impact. There will be lethal climate effects, probably cold and acidic rains. When the climate heats up again, say after ca 20 years.. and with 5-10% human survival planet wide, life will proceed. In the pacific, things go slower.. in 10.000 years or so the Pacific basin may become recognizable again. A new Yellowstone park will form where Hawaii was. A huge, permanently active volcanic area will exist there, rising miles above sealevel. Tectonic effects surrounding this place will make the Pacific coasts more subject to flood and earthquakes.


Following is a link to a site called the Earth Impacts Effects Program put together by some astronomers from Imperial College London and Purdue university. It lets you calculate the effects of meteor impacts upon the Earth by varying metrics including;

  • the size of the object;
  • its composition;
  • the angle of impact;
  • it's velocity; and
  • the type/density of the rock at the point of impact

Earth Impact Effects Program

Details included are the type of damage suffered by people, the local environment and infrastructure based on distance from the point of impact. The point is that by plugging in the size of your asteroid and other parameters I'm pretty sure you'll find underground shelters wouldn't be required once you were a specific distance away from the point of impact. (Depending of course on the standard to which you build the shelters.)

This makes construction of your shelters quicker and easier, so you can have more of them ready in time. Have fun playing with the parameters.

  • $\begingroup$ This doesn't answer the question of homes, so is more suitable as a comment. $\endgroup$
    – Trioxidane
    Commented Jun 11, 2021 at 8:37
  • $\begingroup$ It tells you what the damage to surface structures is likely be according to their distance from the point of impact. Point is for a meteor about the size in question man made surface structures on the opposite side of the globe will survive. So depending on where you are you don't necessarily need to go underground. $\endgroup$
    – Mon
    Commented Jun 11, 2021 at 9:17
  • $\begingroup$ "Following the impact, an earthquake rips through Earth, collapsing any shoddily built shelters. The usual aftereffects of volcanic activity, firestorms, and dust clouds lowering global temperatures. Whereas you could usually go outside soon after everything settles, the "unknown substance" prevents that." It is story locked. I don't think "We're far enough away" counts as answering the question in this case. $\endgroup$
    – Trioxidane
    Commented Jun 11, 2021 at 9:25
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    $\begingroup$ Yes, but obviously (for me anyway) that doesn't stop you building facilities above ground that can survive the impact. The 'unknown substance' stops you going outside, it doesn't stop you living in a sealed environment on the surface anymore than it stops you from living in one below ground. So whats the problem? $\endgroup$
    – Mon
    Commented Jun 11, 2021 at 11:32
  • $\begingroup$ @mon, A 20km rock is going to throw a lot of debris that is going to rain down on the entire surface of the earth for at least a day or two. The heat that will be generated, will bake anybody in a surface structure. The only survivors, will be those who went deep underground, and most of them will die in the first few weeks or months after the impact. $\endgroup$
    – jwdonahue
    Commented Jun 12, 2021 at 3:53

@KerrAvon2055 has demonstrated the political/logistical infeasibility of such shelters, I'm going to take a stab at the technological feasibility.

You have three major problems to solve:

  1. Stockpiling and keeping enough food for 5-10 years
  2. Power generation
  3. Air and Water recirculation

The good news is that we can, indeed, create nutritious food that lasts for up to 10 years. In addition to the usual standards of canned and dehydrated food, MREs can last up to 10 years if stored in a cool dark environment, which an underground shelter would be able to supply. We also already have various stockpiles of durable foods created by various governments and militaries around the world, which would be our initial cache for food in the shelters.

The bad news is that such stockpiles could feed some people for 5 years, but only a tiny minority. For example, even if we assume that global militaries have stockpiled enough food for their troops, it's unlikely to currently be more than 2 years' worth, is likely much less, and that's only for 0.3% of the world population. So you're looking at feeding 0.1% of the global population using existing government stockpiles.

Given 6 months, you can't reasonably produce enough additional food to make a difference. Theoretically, we could create closed-loop hydroponics systems to supplement stored food with grown-in-the-shelter food, but this would increase space and power requirements substantially.

Power generation is ... a problem. Solar would be out because we'd be in for a couple years of heavy cloud cover. Wind would be out because unpredictable storms would destroy it. Geothermal is mostly generated in places you really don't want to be near during a global earthquake.

For fossil fuels, stockpiling enough is a problem. If we assume that 10-person shelter requires as much power as an American house, then we're talking 40,000 gallons of oil to generate 5 years of power -- or four tanker trucks full. And, of course, HVAC on an underground shelter could require more power than that. While larger shelters might have some economy of scale, it seems unlikely that you could go below 1000 gallons of oil per person, and you'd need to obtain, store, and protect that oil from the catastrophe.

Nuclear is therefore your best option, not just to supply five years of power, but also to supply enough power that things like hydroponics and hydrolysis for life support become feasible. For disaster-resistance reasons, though, this requires having small nuclear reactors that could be build underground in a shock-resistant chamber, instead of the large nuclear complexes that are very vulnerable to earthquakes. The good news is that we make such reactors to power ships and submarines; the bad is that very few of these are on land and already built and available for adding to a shelter. And it takes more than 6 months to build one and produce the enriched uranium for it. So that would mean very few shelters indeed.

Finally, air and water is also a problem. To date, humans have not successfully built a closed air or water recirculation system that can work for even one year, let alone five. Again, such systems are under development for the Mars expedition, they are not nearly done yet. And, for obvious reasons, a 5-year system would require at least 5 years of testing to know that it really works. Biosphere 2 showed us how difficult such systems are to design and operate.

If some amount of gas and water from the outside are allowed in (since you haven't defined "unknown substance") things get easier.

So ... if you project this event to be around 10-20 years in the future, it's conceivable that we might have both plentiful mini-nuclear power plants and the air/water recirculation systems to make a 5-year closed shelter feasible. This means that you could use existing stockpiled food to save maybe 0.05% of the population.

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    $\begingroup$ You wouldn't be concerned about the economics in this scenario. You can do Geothermal anywhere. You just have to drill a lot of holes a few hundred meters into the ground. We also know how to build wind generators that would not be destroyed by high winds, they just have a high cost/kwh. But the initial bombardment is likely to destroy anything on the service, so you'd have to stock pile that equipment in shelters, and deploy when conditions allowed, so you would need a large high density energy storage system as well. $\endgroup$
    – jwdonahue
    Commented Jun 12, 2021 at 1:06
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    $\begingroup$ Closed air and water is a problem that was solved back in the 50's/60's. The only reason our nuclear subs ever have to surface, is to restock their consumables and the crews start to get a little stir crazy after six months or so. A large shelter, with well drilling equipment, could drill a new well if the existing well was damaged by the shock wave. While surface conditions may not be habitable for a long time, it would not be long before you could open vents to the surface to bring in filtered air and vent moisture. $\endgroup$
    – jwdonahue
    Commented Jun 12, 2021 at 1:12
  • $\begingroup$ I have now decided that not a lot of people need to survive in the long run since it wasn't possible, and to embrace the conflict before the apocalypse. For my protagonists, however, this kind of project wouldn't be something impossible for money to buy, right? And for a really small portion of the population, the government can successfully create it? $\endgroup$
    – Henry Shao
    Commented Jun 12, 2021 at 2:10
  • $\begingroup$ Sure, either the "reclusive billionaire" scenario or the "rogue government agency" scenario work for this. Don't think of the government as a whole, though; that would break down as goverment/military staff realize that not all of them can survive. Think of who would have access to existing shelters and food stockpiles, like the US Air Force. $\endgroup$
    – FuzzyChef
    Commented Jun 12, 2021 at 4:09
  • $\begingroup$ jwdonahue: I was assuming nothing from the outside because "unknown substance". If the shelter can take in air and water, even if it has to be purified, a lot more becomes possible. $\endgroup$
    – FuzzyChef
    Commented Jun 12, 2021 at 4:10

You can accumulate enough food for several years in six months.


This book suggests tree food.

Cellulosic biofuel production typically already creates sugar as an intermediate product.[28] There are edible calories in leaves, but there is too much dietary fiber, so solutions include making tea, chewing and not swallowing the solids, and making leaf protein concentrate.[29][30] Biomass can be predigested by bacteria so that animals that are poor at digesting cellulose can derive nutrition, such as rats[31] and possibly chickens.

Mushrooms could be grown for several years for food. Humans have a year of food stockpiled, so they'd have a year and a half to get this all ready. People in hazmat suits could manage the fields of crops, and get food for those inside.

You could also grow some low light crops at the equator like potatoes in greenhouses.

You can't build safe structures anywhere.


A collision that hard will cause a magnitude 9 earthquake everywhere, even on the other side of the planet. Even heavily reinforced structures are gonna collapse.

This is a really destructive collision. No near future tech is gonna survive it.

If you didn't have the random floating poison, you could survive it in tents and such, but as is, everything is gonna collapse, and likely everyone will die.


Case 1.

The asteroid actually comes from aliens getting rid of a very large bunch of nuclear/toxic waste by tossing it through a dimensional rift (because that's the environmentally friendly solution, and the local tree-huggers made a lot of noise about how it would not be nice to throw it into the sun, and there was an election, so something had to be done).

-> Everyone dies. But on the positive side, there would be mutants and zombies, so it would at least be entertaining.

Case 2.

The asteroid is a rock.

Then, you already have a bunch of shelters. There will be a huge tsunami where it hits in the pacific, and apocalyptic earthquakes everywhere else.

However, boats in a sheltered sea on the other side of the planet, like the Mediterranean... should be pretty fine. A nuclear powered aircraft carrier with enough fuel rods and spare parts to last a decade or two would make a fine place to watch the world burn. You will need to tow a few cargos full of grain, tin cans, ammo, and maybe a tanker, but it should be alright.

It is also mobile, so when the rest of the world finishes killing (and eating) each other then you can move to the most suitable place that still can support agriculture.

Plus you have plenty of buddies who survived hidden in their nuclear submarines, which means you got plenty of nukes.

So relax, everyone will be your friend...

Of course, if you want anyone to survive, you'd have to make sure the politicians don't. So you still need to build a shelter for them, preferably deep underground, near an "extinct" volcano.

  • $\begingroup$ Anybody on the surface of the planet, is going to cook to death. Well, with the possible exception of Antarctica, where the ice should provide enough cooling to last out the worst of the bombardment, but the sky will be red or white hot, and exposure to direct radiation will cause your close to ignite and your skin to blister within a second or two. Some well built ships, might be able to exchange enough heat with ocean, if they prepare for that scenario in advance, but they are going to be pelted by a lot of molten material, so likely won't float for long. $\endgroup$
    – jwdonahue
    Commented Jun 12, 2021 at 4:21
  • $\begingroup$ And some of those harbors, may experience the tsunami hitting them from shore, before the waves find their way around the continent. It is likely that waves high enough to wash over all but the largest and highest mountain ranges, will scrub the surface of some continents, so many of the "safe harbors" are going to be hit by all that flotsam, and some very turbulent waters. There will be thousands of "rogue" waves forming around the globe, that will rise to a kilometer or more. $\endgroup$
    – jwdonahue
    Commented Jun 12, 2021 at 4:28

Logistics, power and hesitancy is the deciding factor.

I reckon you'll save 10% - 25% of the industrialized world, and no more than 2% of the non industrialized world. Total no more than 400 million people.

To save a population you'll need:

Step 1. Start with a shelter exterior. Either:

  • Dig an underground complex.
  • Dig a big hole and cover it.
  • Find a large pre-dug hole (a mine).
  • Find a natural hole. A cave. Introduce a salt deposit to water. Etc
  • Build a massive concrete above ground structure.
  • Start with existing underground shelters. Car parks. Or subway lines, etc.
  • Dome a city

This is the easy bit. (link to previous answer coming soon) For a previous answer I showed there's a multiple mines in Australia that will accommodate all Australians with their own apartments.

Step 2. Strengthen it.

I don't think you're going to have issues with cost or capability here, most countries can build reinforced concrete or build earth-quake proofing, and have enough spare finance to borrow to cover that.

I think you'll have issues with supply chains (similar to covid19 with early shortages of testing reagents or masks). Only so much rebar and concrete can be made at once, and it can only be moved and cured so quickly.

Step 3. Power it.

5 years of power is hard. Non nuclear nations are going to struggle obviously, but countries with nuclear power are going to struggle to create safe reactors underground with only 6 months notice.

Building the shelter near an underground coal or methane vein, and venting the fumes from an underground generator, is plausible, but a lot of the easier fuel has already been extracted, and that co2 will be there for you when you emerge.

Non nuclear powers could create geothermal power. Setting this up in 6 months is borderline plausible at best.

I suspect the ultimate winner will be reinforced above-ground existing nuclear power plants able to run on near-auto mode. Some flooding protections after Fukashima should help them survive the tsunami.

I expect a few nations will be able to succeed using different methods each, but most will fail and loose life support.

Step 4. Set up life support.

Building underground greenhouses, oxygen recyclers, water filters, etc is a lot of work, but not beyond the capabilities of any large industrialized country.

I don't think anything here is going to trip up most world powers.

Step 5. Move people into it.

Some countries wont be able to literally get their populations to the shelters in time. Australia's 100 thousand buses could get 25 million people to their shelters in 10 trips. With average 7 day round trip to a converted mine and 25 people per bus, that's over 2 months just to get the population to the shelter.

India has 1.6 million buses for 1.4 billion people, that's 4 times the number of trips. Country is smaller but roads are lower quality, assuming average 5 days round trip to get to the shelter, that's 175 days to move the population. That's nearly the full 6 months, not everyone is going to make it.

This also brings up the problem of hesitancy. Looking at those refusing to mask / take vaccine / etc with covid, it's fairly unlikely 100% would support the plan. Many places politicians would refuse to spend money, dooming their entire population.

So who survives?

Europe: Decent chance of getting their act together. Probably using existing nuclear power or fossil fuel deposits. I'd expect to see 25%+ of citizens surviving from western Europe and the UK. Coin toss for each other country.

China: They have big dense cities that could be domed and huge industrial capability and nuclear power right near the cities. If anyone could dome a city in 6 months it'd be China. I'd expect 25%+ of Eastern China to survive.

North America: This will be state or wealth based. All the billionaires will have their own bunker. Some more populous US states (and most Canadian) will be able to build their own bunkers but I'd expect many states (especially those "small government" states) to fail.

Most other industrialized countries have a decent chance.

Most non industrialized countries will give it a go, and some will get lucky, but the odds aren't good.

  • $\begingroup$ Although saving a lot of time, you don't need to find existing holes and caverns. You can be quite liberal with explosives for this. Still slower, but a great alternative as the natural or already dug holes aren't in abundance. Still, I think the quoted saving of 25 and 2% are way too high. $\endgroup$
    – Trioxidane
    Commented Jun 11, 2021 at 8:56
  • 4
    $\begingroup$ @Trioxidane Project plowshare. Nuclear excavation. But dome a city? In a way hardend against this kind of impact? No, we don't have the capability. Dome's don't scale like that. $\endgroup$
    – Yakk
    Commented Jun 11, 2021 at 18:45
  • 1
    $\begingroup$ Some good info in this answer, some not so good. Dome a city? Seriously, dome small areas maybe. Your assessment of the US is off, the US is highly federalized now, there are no "small government" states, and an all out effort such as this would be highly federalized. Indeed, it would be the higher population states that would face more difficulty with the greater logistical issues of providing shelter for huge populations. Not to mention larger populations of 'at risk' individuals. $\endgroup$
    – Glen Yates
    Commented Jun 11, 2021 at 19:30
  • $\begingroup$ For thermal power generation, you just need to drill a few hundred deep wells (100..300 meters) to extract a megawatt of power. They can be constructed in a matter of a few months if you forgo the permitting process, but the price/KW is still very high, due to the cost of the equipment, which requires low boiling point working fluids to run purpose built turbines. The key is, you can build these in existing mines, or just about anywhere, you don't need to be near a volcano to extract usable power levels. $\endgroup$
    – jwdonahue
    Commented Jun 12, 2021 at 4:11
  • $\begingroup$ Come to think of it, the worlds deepest mines would make good refuges and their lower parts are already quite hot, so you could just install heat pumps the lower areas, and drive the turbines near the upper areas. Of course, until the atmosphere cools down a good bit, after the impact, you won't be able to use these systems if there's insufficient nearby, and cool enough aquifers, to dump some heat into. $\endgroup$
    – jwdonahue
    Commented Jun 12, 2021 at 4:16

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