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In many survivalist stories the intrepid heroes wisely (given the impending plot line) decide to build and stock a fallout shelter. When the inevitable catastrophe comes they move into their underground bunker and live in relative comfort while the bulk of humanity perishes in the event and aftermath. An underground bunker makes good sense for threats like bombs or other large general threats, but it seems like that is all an underground bunker can reasonably defend against.

Underground bunkers that even a wealthy survivalist could build appear to rely on a strong door and concealment for survival against assault. That being said the strong door isn't a true deterrent since the air vent/intake must be exposed. If that air intake is blocked or restricted with mud or a plastic bag and some duct tape the only option for the inhabitants is to sally forth. Given the extremely limited egress points from the shelter these are easily covered by a small assaulting force.

So how can an underground bunker be reasonably defended against roving bands of bandits? Especially considering the cost of blocking an air vent/intake is exceedingly low compared to the cost of defending the air vent/intake. This is the opposite of a defensible position. What tweaks would be needed to make an underground bunker defensible?

Key assumptions:

  • Current Tech
  • No access to extreme amounts of power like a nuclear power plant in the bunker.
  • Completed bunker must be affordable enough and reasonably attainable for a wealthy survivalist or a small group of wealthy survivalists
  • Location is rural continental United States
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    $\begingroup$ If they can grow their own food, they can close carbon and oxygen circles and not need air intakes. On the other hand, if they live on stockpiled food, CO2 scrubbers and oxygen tanks can go a long way. What time frame are you looking for? $\endgroup$
    – Mołot
    Dec 4, 2017 at 23:53
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    $\begingroup$ @Mołot yes but that involves huge amounts of constant power. This was tried in Russia but they had a hydroelectric dam to feed it power. $\endgroup$
    – Erik
    Dec 4, 2017 at 23:54
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    $\begingroup$ actually nuclear power plant in cargo container is current tech. Just so you know. $\endgroup$
    – Mołot
    Dec 5, 2017 at 0:02
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    $\begingroup$ What do you mean "the" air intake? Why would you think there would only be one? $\endgroup$
    – TMN
    Dec 5, 2017 at 13:21
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    $\begingroup$ Hah from the title I thought it was an ethical question! Like "is it morally defensible to hide in a bunker while the majority of people die?" $\endgroup$
    – Zanna
    Dec 6, 2017 at 11:40

7 Answers 7

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Much of what you are asking has been answered in the 5000 year old recorded history of warfare. Essentially, what you are asking about is how to protect your fortress or fortification from investiture or siegecraft, with a specific set of conditions.

The first rule of fortifications is that there should be no single point of failure. There are always at least two gates, and possibly more "sally ports" to allow your forces to exit and counter attack the investing forces.

Similarly, the air handling system of your shelter is also multiply redundant, with several concealed intakes and exhausts, widely dispersed to prevent accidental or deliberate interruption of the air supply. You need this anyway in a nuclear shelter in order to shut down parts of the system to change filters in order to prevent the entry of radioactive particles or other contaminants into the system. In fact, a well designed and built shelter would probably have geosensors, cameras etc. to detect both movement on the ground and digging in and around the area of the bunker, much like castles in the middle ages had "countermines" to allow defenders to hear attempts to dig under the castle walls.

Larger and more elaborate shelters would also be divided into separate compartments which could be sealed to isolate damaged, contaminated or compromised sections, and a very well stocked system would have equipment on hand to dig either up or out.

enter image description here

Diagram of NVA tunnel and shelter system

Only a hastily built and poorly planned shelter would lack these features (perhaps the survivalist simply buried a 20' ISO container in a trench as the basis of his shelter), but unless there was absolutely no time or resources, I suspect that a person thinking along these lines would gradually add to the shelter and incorporate these features over time.

History tells us that a manned and defended fortress can hold out for a considerable period of time, provided the defenders have sufficient supplies. Even in the modern age, fortifications could be blasted to rubble, which simply provided the defenders even more concealment, and made attacking a nightmare. With sufficient preparation, extensive tunnel networks can be built to make attacking them nightmarish, as the Americans discovered in the Viet Nam war, or the Israelis have discovered in Gaza and Southern Lebanon, and these have stood up to serious bombardment by heavy artillery and large bombs. Either dedicated bunker busters must be used, or actual engineers enter the networks with tools and explosives.

enter image description here

IDF soldier in underground tunnel in Gaza

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    $\begingroup$ This is a great point to study the NVA instead of the advertisements for the companies selling pre-fab or other commercially offered shelter systems. A blend of the battle tested cheap NVA designs with the capabilities offered by some of the commercial vendors seems like a great solution. $\endgroup$
    – Erik
    Dec 5, 2017 at 19:03
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    $\begingroup$ The US eventually ended up pumping propane and oxygen into the tunnels and turning them into one massive air-fuel bomb. It sucked all the air out of the tunnels. $\endgroup$
    – Justin Thyme
    Dec 5, 2017 at 21:58
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    $\begingroup$ As soon as the threat from fallout passes, our regularly scheduled program may resume: War Never Changes. $\endgroup$
    – Mazura
    Dec 5, 2017 at 23:50
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    $\begingroup$ just a nitpicking, but countermines were a feature of 17th-18th style century fortification, not the middle ages ones. $\endgroup$
    – Edheldil
    Dec 6, 2017 at 15:49
  • $\begingroup$ Thank you. I had exactly this book image in mind when I answered. $\endgroup$
    – James McLellan
    Dec 7, 2017 at 11:42
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Yes, if they are unapproachable.

swiss bunkers on cliff face https://www.newlyswissed.com/inside-a-secret-military-bunker-in-switzerland/

Site your bunker in a cliff. One must climb a ladder to the entrance or rappel down, possibly from a considerable height. It is easy to defend against attackers swinging in the open in front of a cliff face.

Even if they use cannons to bombard your front door until it is a hole in the cliff, you will have a second door set well back inside the cliff. It is easy to defend a stone hall people must walk down to attack your second door.

Site your air intakes on the cliff face. They will be impossible to find among the natural irregularity of the cliff face.

You might be at risk for attack by persons (very dedicated persons) tunneling down from above. Put your bunker under 500 meters of granite to slow down tunnelers.

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    $\begingroup$ @bendl that's only a problem if cliff faces nearby military installation or a city. Chose a cliff that does not. $\endgroup$
    – Mołot
    Dec 5, 2017 at 7:46
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    $\begingroup$ @bendl I'm pretty sure you'd shake everybody in the bunker so violently, they'd just flat out die. $\endgroup$
    – Clearer
    Dec 5, 2017 at 10:36
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    $\begingroup$ @Clearer Few nuclear shelters are made to survive a direct hit. Mostly, they are about surviving a distant flash, atmospheric shock-wave and the subsequent fallout. Given that the shelter is at a distance from any significant target, no bomb will fall close enough to really shake a well-built shelter. Unless you use it to survive a meteorite strike and are very, very lucky about where it falls. $\endgroup$
    – Eth
    Dec 5, 2017 at 13:39
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    $\begingroup$ @Clearer How strong would the shaking be, in comparison to an earthquake? Earthquakes don't kill people, falling debris and collapsing homes do. The bunker should not collapse in such an event $\endgroup$
    – phflack
    Dec 5, 2017 at 14:13
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    $\begingroup$ But now you're very vulnerable to sieges. People can't come in at all, people can't come out at all. Post 3 people on top of your bunkers and below, and you're starving to death with minimal effort from the other side. Great against direct attacks... Not so great about simple strategy $\endgroup$
    – Nepho
    Dec 5, 2017 at 14:38
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You can vent into a cave system. Particularly doable in areas with very large cave systems, such as Kentucky and parts of Texas. Many cave systems are not fully mapped, so if your survivalists are spelunkers, they might have a fall back option into an area where they hold all of the information advantage. For urban survivalists, one of the many forgotten undergrounds of major cities : Atlanta, San Francisco could be adjacent to the bunker - same information advantage as a cave system.

You can vent into crevasses, which may be impossible to reach without blasting, and also may diffuse any signature. German bunkers vented into old forest where overgrowth did a good job of hiding the exhaust. Or, you could choose terrain that is just frighteningly inhospitable for the would-be invader (see Snake Island), or riddled with possibly natural booby traps against excavation (see Oak Island), or almost impossibly remote such as the deep deserts of the Midwest, that really require both a good water supply and knowing where your going.

In coastal areas, or mountain areas it's possible to build such that the bunker is naturally sealed by water (think a beaver dam, or supposed German caches in Corsica) during flood or high tide, and only open during dry seasons or low tide. Or, always flooded (vented into an inaccessible small cave system).

I've seen a few episodes of Preppers where the primary bunker has several caches/spider holes around it, for the purposes of the bunker-borne to circle around wannabe intruders and catch them by surprise.

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  • $\begingroup$ Is "deep deserts of the Midwest" supposed to be "of the Southwest"? Assuming you mean the Midwestern US, about the only thing that might be considered a deep desert here might be the Badlands of South Dakota. $\endgroup$
    – bcrist
    Dec 6, 2017 at 2:34
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Nobody is going to stop the tweakers. Millenialist survival super wealthy guy built a large facility here (somewhere close to https://goo.gl/maps/8sEHhrpzdEJ2 I think that was his landing strip) in Southern Oregon (farm, stables, school, generators, water power) and at it's heart was an underground home with two sets of inch and a half steel doors. After completion he and family went away for a week and came home to a completely looted home. The thieves had brought in welding rigs lowered by rope to the entrance and found every secret hidey hole within it taking guns and gold. I got to see it while helping a family member who was asked to check it for water leaks and other maintenance. I was shocked that they could find all those secret rooms and compartments. I asked and was assured that it hadn't been an inside job by one of the builders. Just go down to Harbor Freight and check out all the breaking into places tools available.
There were niches for defense, to shoot through, but it was easy to see how these could have been readily defeated by lowering protective plates to shield the welders. The inner "safe rooms" were engineered similarly to the entrance and broken into the same way.
What I learned is that the more elaborate the defenses, the more enticing the target. Perhaps a false target above (a secure building with lots of good stuff) would have helped but I think those tweakers would have found anything.
I think the best plan is absolute secrecy and a humble design, no shiny steel or other indication that there's good stuff inside. Plenty of old mines around all over the country, plenty of dogs to let you know if strangers are about. A rusty lean-to or tent above ground to explain any observed occupation of the area ought to do you.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – HDE 226868 ♦
    Dec 8, 2017 at 3:06
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If your survivalists are truly wealthy, then I would posit they should go under water, as in a very large lake (ideally, one of the great lakes), not under land. With the availability of personal submarines, it is feasible for the air to be retrieved by remote submarine vehicle which could surface anywhere in the body of water. Access could be restricted entirely to under water. The water would be a good barrier against radioactivity. With good sonar, approaching it would be detectable. Not to mention that anyone wanting to penetrate it would have to be high tech.

Assuming enough wealth, then torpedo-type weaponry could be used for defense.

The advantage of under water is that it nullifies so many traditional attack weapons. Guns, missiles, lasers, bombs are useless. Any siege would have to be prolonged, high tech, and concerted. Siege boats would be siting ducks (literally) for the defenders. Very difficult to bring in any excavating machines. No drilling rigs.

A water supply would not be an issue. Mini nuclear power plants are available for small-scale power, and it is feasible to extract oxygen from water, if the facility is large enough. Algae beds, for instance. Geothermal energy.

The water temperature would be relatively stable. Environmental climate changes would not be a factor. No hurricanes, storms, tornadoes, electrical storms.

Perhaps this would generate another OP - 'What would be an effective way to penetrate an underwater high tech fortress?'

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  • $\begingroup$ Such a facility would require several highly skilled techs (and their families or at least a playmate - who's going into the apocalypse without a date?). Without outside support don't know if you could break even between number of people and expanding the facility to support them (and do it secretly? no way, nowhere). Access points will always be weak point; adding complexity through submarines weakens it. Sophisticated weapons? going to need more people, experts (and their dates). What happens when one of the expert's kids goes into insulin shock and he wants to take a sub and find help? $\endgroup$
    – Hebekiah
    Dec 7, 2017 at 19:39
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    $\begingroup$ We have a Be Nice policy, Justin Thyme. Please stick to it. $\endgroup$
    – HDE 226868 ♦
    Dec 7, 2017 at 22:10
  • $\begingroup$ @Hebekiah I would foresee that, in a very short time, the ability to maintain such a complex would be turned over to automatons and automation. Take automobiles. A mere decade ago, they required constant pampering and maintenance. Now, the maintenance schedules are in the 80,000 km. range, and on-board diagnostics are auto-correcting. I would posit that it is not unreasonable to expect such a facility would not require techs to maintain. Computers would be able to detect exactly what modules need to be replaced, and the swapping would be a simple matter. $\endgroup$
    – Justin Thyme
    Dec 8, 2017 at 2:24
  • $\begingroup$ @HDE 226868 Do you also have a 'no floccinaucinihilipilification' policy? Yes it is a word. Yes it is applicable. $\endgroup$
    – Justin Thyme
    Dec 8, 2017 at 2:27
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    $\begingroup$ @JustinThyme I am not referring to that word; I am referring to you calling other users "trolls" and describing their actions as a "narcissistic rage". That is not acceptable. $\endgroup$
    – HDE 226868 ♦
    Dec 8, 2017 at 2:59
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Well if your main concern is air supply, you could have a pipe with lots of little holes in an area, covered with loose stones. I think I saw that in an ad for a zero-energy house. Or you can have a closed system. Getting your energy from geothermal energy And get from the heat electricity via water steam or a low boiling point liquid like aceton or thermocouples (I don't know if geothermal energy is a realistic way to power a house or bunker. Maybe if you have hot springs nearby. Or even an active volcano. But I think that goes against the no extrem amount of power.) Photovoltaics can also be stolen or covered up. As with wind turbines.

Btw the problem if the vents get blocked is the CO2, since its concentration will rise faster in dangerous high levels than the oxygen concentration reach dangerous low levels. Submarines use CO2 scrubbers for that. They need to get refreshed. Depending on the system, they just need to be heated up. That releases the CO2 again.

Of course if you get sieged you want more exits, that nobody knows about. Basically secret exits. The same for the air vents. Have secret ones.

I think it is possible to get a bunker that can have the air intake cut for 30 days or longer, for the price of a middle class house. So it is just the question if the attackers can open the door. If not they would need to wait until the people inside left. If they pass out inside (and die), the attackers wasted time and don't get anything without opening the door. On the other hand you want to attack your attackers somehow or they will be able to break open the doors/wall with a pick axe.
If the attackers are looters and can't have a safe base near the doors, they would leave probably after a day or maybe in hours. I heard thieves skip your house, if they can't get in in a few minutes.

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  • $\begingroup$ You may want to define the cost better, a middle class house where I live is about 250K, a middle class house near Philadelphia is about 400K and it only gets higher. $\endgroup$
    – Reed
    Dec 5, 2017 at 14:27
  • $\begingroup$ Well, that's the problem. How big should it be. How much energy would it need, what furniture and so on. Where is it built. I found that a geothermal heating pump for an average household would be about 25k, but that would only help with heating. I still don't know if there are private geothermal power plants that produce electricity. There are industrial ones, but I guess they would be significant more expansive. $\endgroup$
    – Anakin Veganos
    Dec 5, 2017 at 15:48
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    $\begingroup$ Geothermal costs $2-5M per MW: en.wikipedia.org/wiki/Geothermal_power#Economics $\endgroup$
    – icc97
    Dec 5, 2017 at 16:07
  • $\begingroup$ @icc97 $5/watt is a pretty routine price for any type of power plant these days. Swapping a 40W incandescent for a 5W LED is far cheaper than the debt service on a new power plant to make the 35 watts. Once you make the mindshift to see it that way, there's great gobs of money available to pay for efficiency. That's how Walmart has bulk LED bulbs for a buck each, they are subsidized by the power companies because it's the cheapest power plant they can buy. $\endgroup$
    – Harper - Reinstate Monica
    Dec 6, 2017 at 0:29
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    $\begingroup$ @Harper I guess my underlying point was that it's unlikely for a 'reasonably' wealthy individual to afford to pay for $5M+ power plant (and it certainly invalidates the assumption of it costing the same price as an average house). I'm assuming that it's on the high side of the range because these estimates aren't made for people wanting to build a power plant in their bunker. But the bunker will need power from somewhere. $\endgroup$
    – icc97
    Dec 6, 2017 at 8:32
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IMO the only real answer to this question is to make it a closed system. If you're growing your own food you're already a good way there. If that's not enough, then make some more oxygen yourself by splitting water into hydrogen and oxygen.

The other solutions all suffer from various problems; if it's very good against nuclear blasts then it's not good against a trash bag and if people can't thwart it then it's likely not safe against nuclear. Not to mention that if you are thinking about a nuclear attack then the outside world will be irradiated, and a closed system will really be your only option.

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    $\begingroup$ Splitting water into hydrogen and oxygen is easy only if you have a cheap power source that doesn't use oxygen - and what are you going to do with all that hydrogen? A closed ecosystem has been attempted many times in modern history, and we didn't even get close. It's trickier than it sounds even for bare survival. And carbon dioxide buildup kills you long before the lack of oxygen - you need some way to scrub the carbon dioxide and feed it to your plants/algae. And we don't really have a good sustainable solution for that either (usually we just dump the carbon dioxide). $\endgroup$
    – Luaan
    Dec 5, 2017 at 14:51
  • $\begingroup$ A valid point, but perhaps I should amend my answer to say "mostly closed". What comes out of the underground base is irrelevant, and some things can come in if they're not exposed to the surface. For instance, build it over an aquifer and take as much water from it as you want. Make it a pressurized aquifer and you get some hydroelectric power! Put it together and you get oxygen. As for the excess hydrogen that can be vented however you see fit. $\endgroup$
    – bendl
    Dec 6, 2017 at 16:39
  • $\begingroup$ @bendl. Pressurize it, store it in tanks far from the base, and use it to set things on fire more effectively. $\endgroup$
    – Mad Physicist
    Dec 6, 2017 at 22:15
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    $\begingroup$ @Luaan: Simple systems don't seem that tricky...dailymail.co.uk/sciencetech/article-2267504/… $\endgroup$
    – DJohnM
    Dec 7, 2017 at 7:20
  • $\begingroup$ @DJohnM Yeah, but that's not quite what we're talking about here, and it still relies on an energy input from the outside (the Sun). A closed system that can sustain a human being is quite a different challenge entirely. The large-scale experiments with that have been unsuccessful so far, and by a large margin - and that's talking about very large installations that require direct exposure to sunlight, not an underground bunker. We're just nowhere near that capability, even today. $\endgroup$
    – Luaan
    Dec 7, 2017 at 18:13

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