If I had a huge underground bunker powered by generators and the generators failed, would it be warm enough underground to keep people alive? How deep would it have to be for people to be comfortable without a heat source?

[Edited to remove second question - I have solutions for the air recycling and stuff like that it’s just the temperature I’m thinking about here.]

  • $\begingroup$ I think it may lack a bit of detail : what kind of generators, how thick are the wall, is there other way to provide light than electricity. I think you need to be more precise ! $\endgroup$
    – Cailloumax
    Jul 17, 2019 at 13:52
  • $\begingroup$ Most cave systems stabilize at the average annual temperature of the surrounding surface area, with very little variation inside the caves. Typical are mid-50s F with +/- 2F. Most people would find this a little cool, but not cold, and easily mitigated with coats and blankets. $\endgroup$
    – cobaltduck
    Jul 17, 2019 at 13:53
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    $\begingroup$ The moment the generators fail in my subterranean base, my concern is more about air than temperature. Especially if there are 149 other pumping precious oxygen at the same time. (But then, not the topic of the question, though it has to be considered) $\endgroup$
    – Nyakouai
    Jul 17, 2019 at 14:10
  • $\begingroup$ Welcome to the site. Please note that we strongly encourage users to wait at least 24 hours before accepting an answer. We have users all over the world and accepting early may discourage other, better answers from appearing. $\endgroup$
    – Frostfyre
    Jul 17, 2019 at 15:27
  • $\begingroup$ "Also, what sort of things would be needed in a self sufficient bunker of around 150 people - food production, cooking areas, etc" Is a separate question. Please remove that part from here, and ask that in a different thread so each question be addressed individually. $\endgroup$
    – Nosajimiki
    Jul 17, 2019 at 17:56

3 Answers 3


Usually one question per question. I'll answer the temperature one.

Unless you were very close to the poles, and not too deep, it would be shirt-sleeves livable with or without a power source. Maybe not exactly comfortable, but people would not be dropping dead of cold as long as they had a light jacket.

Scroll down this article to item 12 and you will see that Carlsbad Cavern is a very modest 56°F year round. In Mammoth cave this article tells us it stays around 54°F. Closer to the poles the temperature will be lower.

This Wikipedia article explains how temperature varies with depth. It's complicated by location near various things like volcanoes and plate boundaries. But as a back-of-the-envelope calculation we have the following.

Away from tectonic plate boundaries, it is about 25–30 °C/km (72-87 °F/mi) of depth near the surface in most of the world.

So probably you would be more worried about not being too deep when the power goes off, because your cooling would stop. If you were, say, 3 km deep, you'd be getting pretty close to boiling without some A/C working.

  • 2
    $\begingroup$ If the question is too broad because it has more than one question per post, it is better to request the OP to fix the issue instead of dropping an answer. $\endgroup$
    – L.Dutch
    Jul 17, 2019 at 16:10
  • $\begingroup$ This answer is also incorrect since it it talking about temperatures in naturally occurring caves, which are different than the "underground base" mentioned in the question. $\endgroup$
    – krb
    Jul 18, 2019 at 14:16

About two feet below the local frost line is more than sufficient. Humans generate an incredible amount of heat, and you'll probably be more concerned with dissipation of that heat. Per the other answer which references things like Mammoth Cave: since you will be walling off your living area, you don't deal with the huge open cave spaces nor the airflow in them.

Maintaining a good air supply (and access to fresh water) are more important.


Temperature would be the least of your problems in an underground bunker with no power supply, blankets for too cold, spray of water for too warm. But remember, CO2 is heavier than oxygen, so you need well defined measures in place to take care of the air recycling. Plants can do it for you but only if there is artificial sunlight. So if your generators go down you will suffocate to death. You can also find some bacteria or algae that can help, especially the ones that don't depend on light. But I am not certain of the viability of this method.

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    $\begingroup$ Or be smart and think about how you’re ventilating the bunker ahead of time. A well designed bunker can use natural airflow (wind) or mechanical power from water sources to pull fresh air straight into the tunnels. Heck, for centuries the predominant form of mine ventilation was a well placed fire! $\endgroup$
    – Joe Bloggs
    Jul 17, 2019 at 18:02

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