In my world (a very soft sci-fi world), humans live beneath the surface in a large planet that is largely inhospitable on the surface. It's not Earth. The hollow planet is kind of like Swiss cheese in that it's mostly solid but there are large "burrows" where humans live and they connect to one another through large tunnels. 300 years prior in this world's history, a burrow launched a nuclear bomb at another burrow during a war. My idea is that after this, the burrow that had a bomb dropped on it was sealed off out of fear that the nuclear waste would travel to the rest.

Is this a plausible reaction? My line of thinking is that since they are underground, there are less places for the fallout to go compared to a planet where the people inhabit the surface. In addition, would sealing off this contaminated burrow even prevent further damage from reaching the other burrows?

Additional information: this bomb was around the beginning of the world's foray into nuclear technology. Its strength was 21 kt.

On average, the distance between these burrows is about 3,500 miles. The tunnels that connect them are not straight. They curve, sometimes moving up and down.

There is volcanic activity on the surface, though not a lot above the burrow that suffered the attack.

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    $\begingroup$ Depends on: Size of the bomb (in KT or MT), the distance apart of the burrows, the nearby tectonic stress, the possible presence of volcanoes, the consistency of the local rock.... etc. Could we have a few more details. $\endgroup$ Jul 23, 2021 at 1:35
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    $\begingroup$ Frame challenge 1: One would think that a 21 kt blast would open the entire target burrow to the atmosphere, venting out most of the fallout. $\endgroup$ Jul 23, 2021 at 2:39
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    $\begingroup$ Frame challenge 2: In theory, the blast could travel along the connecting tunnels but: 1. It is extremely implausible that multiple 3,500 mi. tunnels between burrows would be kept pressurized and habitable all the time. The energy and maintenance cost would be absurd. 2. Even if the tunnels are somehow pressurized, it is also extremely implausible that there are no safety doors present along the 3,500 mile tunnels to handle pressure loss caused by accidents or earthquakes or to stop fire from spreading or to deal with flooding. These doors will stop the shockwave. $\endgroup$ Jul 23, 2021 at 2:41

5 Answers 5


It would be much less disastrous.

Nuclear bombs are deliberately detonated high above the surface because then they can cover more surface area with direct destructive heat, pressure, and radiation. Underground all the tunnels and twists and turns would quickly deplete the energy of the nuclear bomb.

Air is much easier to travel through than rock, and it's much easier to nuke someone through air than rock.

If you want nuclear fallout to spread you need wind to move through the tunnels carrying radioactive material. It's certainly reasonable that such wind could carry radioactive materials, although at a distance of 3500 miles the radiation would be negligible. It might be good to have a satellite settlement near the nuked one that could potentially be hurt, within a mile or two.

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    $\begingroup$ That being said, for those areas affected, the blast is far more intense and devastating $\endgroup$
    – Hobbamok
    Jul 23, 2021 at 14:49
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    $\begingroup$ As a result of being detonated high, there's actually very little fallout/radiation. Fallout is nuclear material attaching to dirt. If you detonate high, you're essentially flattening the ground below with the shockwave, but not causing radiation to hit the ground, thus not creating fallout. So you would need to both detonate near, on, or in the ground, and have wind (as you stated) to carry it. (source, Dad was an ICBM missileer) $\endgroup$ Jul 23, 2021 at 18:09
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    $\begingroup$ The nuclear bomb itself is converted into radioactive material, and can spread a huge distance. While a nuke encased in earth will produce a lot of radioactive material around it, it won't spread, as I noted in my question. $\endgroup$
    – Nepene Nep
    Jul 23, 2021 at 21:56

They adapted, developed a divergent culture, and physically modified themselves.

When a nuclear bomb drops near your house, it's time to consider moving. But when a lot of people want to move, they're called refugees, and the countries of the world join together to help them with walls, deportations, and a finely nuanced appreciation of every conceivable risk they could bring.

Trapped in the fallout zone and left to their own devices, the residents suffered an elevated cancer rate as the short half life elements decayed. There are areas with high background radiation on Earth that are inhabited, and just how people seem to adjust remains a bit mysterious. That probably only goes so far, and I don't think that a site would recover nearly as well as Hiroshima in an underground bunker situation because would seem to be little ocean on hand to receive contaminated runoff.

Let's suppose, then, that they simply used other nuclear-age tech, namely genetic engineering, to address the cancer problem directly. We might consider what would have happened if He Jiankui explored the use of p53 paralogues to prevent cancer as in elephants, instead of creating a possibly hazardous variation to resist HIV.

After a few generations, the fallout from the weapon is long since past. But the society is made up of people who resist cancer, and have customs and standards to match. Getting a radiation burn from tinkering with the reactor is still a problem for them, but it is as mundane an issue as burning oneself with a stove. You can imagine then that they would make relatively free use of energy sources and enjoy a higher overall standard of living, yet their agricultural products would not be fit for "human" consumption.

Beyond fear of their products, there would also be fear of the people. They are now culturally and genetically different, and have been cut off for generations. Allowing some of them into your country, erm, cave means that you have residents who might feel free to contaminate their home, the town, or the entire cavern, knowing that their people alone would then be able to live there. The social fear and persecution would be more intense than any such fear we can recall having in our own history in regard to peoples who are functionally genetically identical on average.


No real problems.

Yes, some radionuclides might travel even a long distance, but without active air circulation diffusion times are very, very long. And only apply to volatile isotopes, so mainly 14C, 18O, a little 15N, some neutron-enriched argon; in practice, they're all innocuous. The real stinkers - 141I, 137Cs, 90Sr, don't move so much. All the rest (from irradiated common rocks, dust and organic matter) are pretty unstable isotopes, and decay below the danger threshold within two to three months. In absence of wind, they won't travel more than a few tens of meters beyond the immediate ejecta range (which, depending on the size and shape of the tunnels, might be considerable).

The blast will travel quite far, I'd expect a hundred to three hundred kilometers due to the "rifle barrel effect". Expect massive damages within 20-30 kilometers, depending on the burrow size and geometry. Hypothetically, the burrow ceiling might crack or even crash down, burying the burrow, again depending on burrow position, depth, nature, local geology and so on. There might be some fracking, some repositioning of water tables and slipping of nearby fault lines.

3500 miles away, no problems whatsoever.

But this is me. The people in the burrow might not be familiar with radiations, and FUD (fear, uncertainty, doubt) is a force in itself; just look what happened to Japan's hibakusha. Sometimes their children get shunned.

From a burrow administrator's point of view, it could be faster, cheaper and more expedient to build an impenetrable wall cutting off the affected burrow - after all, they might all be dead and they aren't their citizens anyway - than to explain all of the above to their constituency.


You can do better than nukes!

The thing about a real nuke is that the fallout is only dangerous for a while. Things turned radioactive by a nuclear explosion turn unradioactive after a short time. Those tunnels would be full of dusty debris.

Nukes. Flash boom. Ho hum. You have a sweet tunnel world! Soft sci fi it up! Make their bomb something interesting! I am picturing a weird energy release that was hungry for life and that changed and fed on things in its path. Purple tendrils and maybe you can hear them in your mind. Things came up from the utter deep to join them.

You need to block off those tunnels with serious seals and not for theoretical reasons. Very bad things are in there.


21 kilotons is small

21 kilotons is (probably deliberately) the estimated yield of the Fat Man device used against Nagasaki in World War II and almost the same as the preceding Trinity test of a 22 kT device. Physically it was a very large device which was designed to air burst at an altitude of 500 m in order to cause damage to surface structures over the maximum area.

If the equivalent of Fat Man was literally "dropped" by something that could survive flying above the inhospitable surface then it would cause negligible damage to a deep underground structure. At Nagasaki the radius of total destruction was approximately 1.6 km with extensive fires out to 3.2 km as a result of the airburst. However, there was no crater. The Trinity test resulted in a crater only 1.4 metres deep with a radius of 80 metres. Shallow structures directly under the blast would certainly suffer, but any solidly-built structure over a hundred metres deep would survive easily.

The Fat Man device was relatively fragile - even if there had been a strategic requirement for such a weapon, it would not have been possible to build a warhead that would penetrate into the ground before detonating. However, if by some means the bomb could be either dropped down a vertical shaft that led to the heart of an underground city (after, of course, maneuvering down a trench protected by turrets and TIE fighters) or smuggled into a deep underground city - what then? Fortunately we have the answer as a result of the various underground nuclear weapons tests that were conducted after nations working on nuclear weapons realised that spreading fallout over large areas downwind of their test sights annoyed the people living downwind.

Underground effects are proportional to the cube root of the yield in kilotons. To get some nice clean numbers, let's work with a 27 kT weapon (so we have a cube root of 3) instead of 21 kT.

  • Melt cavity formed by the explosion is 55 x 3 = 165 m radius
  • Crushed zone, where rock has lost all integrity is 2 x melt cavity ie 330 m radius
  • Cracked zone, where rock has radial and concentric fissures = 3 x melt cavity ie approximately 500 m radius
  • Zone of irreversible strain is 200 x melt cavity ie approximately 5 km radius
  • Surface collapse (ie crater form on surface as material collapses into melt cavity) - 95% chance if detonation depth is <450 metres, approximately 50% chance if detonation depth is >540 metres

The above numbers depend on the type of rock that the underground burrows are built in, but the takeaway lessons are:

  • An air burst will make the surface even more uninhabitable but will not affect a durable underground burrow at all.
  • A surface burst will create a very shallow crater and will only seriously damage a shallow-depth underground burrow. It will make the surface even more uninhabitable by throwing a very large volume of fallout into the atmosphere.
  • A deep underground burst will create structural damage out to 5 km from the centre. However, it is likely that the structural damage will collapse any tunnels leading out of the affected area and therefore it is unlikely that significant amounts of radioactive material will contaminate surrounding tunnels.
  • 3500 km away there will be no effect underground. If there was a surface burst and they are downwind and they are monitoring for fallout on the surface then they may find something.

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