I'm trying to build a setting in which humankind would have no other choice than to live underwater to survive.

Suppose there was a nuclear war in which a huge part of the human population died, and it renders the surface of the earth uninhabitable for several centuries. But people survived because they were conducting an experiment at the time, living in underwater habitats.

How would this fallout then affect the oceans? I'm especially interested in how much the water would be impacted by radiation, if it would affect the edibility of flora/fauna, and if this could possibly cause mutations.

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    $\begingroup$ Welcome to the site neon, I made some edits for readability (didn't change any content). Check out the help center and tour when you get a chance to get familiar with the site. $\endgroup$
    – James
    Aug 22, 2017 at 19:57
  • $\begingroup$ If you want to force people to live underwater, getting rid of all the land might be a good start. $\endgroup$
    – barbecue
    Aug 23, 2017 at 15:22
  • $\begingroup$ Advise you to read Seveneves, by Neal Stephenson. $\endgroup$
    – Tin Wizard
    Aug 23, 2017 at 18:25
  • $\begingroup$ if its a bit more far-future, maybe use some orbital x-ray laser weapons that sterilize the surface? And these are solar powered and still active, and now civilization has lost the ability to launch rockets to take them out. Of course, you could live in caves or build shelters with thick roofs, but going underwater might be a viable option, especially if you base the food chain around underwater fish and kelp farming $\endgroup$
    – Innovine
    Aug 25, 2017 at 9:15

7 Answers 7


Your premise is problematic

I am going to take two steps back and start by looking at what you want to achieve: a way to force humans to live under water.

Nuclear fallout does not really help you there, because of the way fallout works. Fallout is not some evil magical gas that floats around and makes surface life equally problematic everywhere.

In short: nuclear fallout is dust, not gas. The problem for you as an author with the fallout premise is that fallout does just that: fall out of the sky, as soon as there is some rain. That is how it gets its name, it simply falls out of the sky really quickly. This means that it is the surface that gets contaminated, not the atmosphere. And this contamination will not be uniform... it will instead be very uneven and spotty.

Nuclear fallout is harmful in two ways.

1) Direct radiation, we can call this "shine". 2) Ingested particles; i.e. dust particles that get into your body by breathing or eating.

Both 1 and 2 are solved in the most banal of manners: remove the dust, mechanically. Simply scoop it up and lift it away. Once that is done, prevent new dust from getting into your cleaned areas.

Doing this — cleaning up contaminated areas and make them fit for living and sustenance — is a lot easier and cheaper than moving down into the sea.

As evidence: this is how Hiroshima looks today...

enter image description here

So in short, you need another premise... and there is one:

The ozone layer is destroyed

The ozone layer is essential for us humans to live on the surface of the Earth. But even with the protection that the ozone layer gives us, ultraviolet radiation from Sol is still one of the leading factors to increase the risk of cancer(1), and very bad cancer at that: malignant melanoma.

The ozone layer absorbs 97 to 99 percent of UV radiation. So without the ozone layer, we could suffer 30 to 100 times more UV radiation on the surface of the Earth. This would pretty much preclude humans spending any time out in daylight.

Destroying the ozone layer is "easy" for you as an author. Remember that it nearly happened in real life. It was just that we caught the problem in time and fixed it. All you need to do is introduce a factor that does lead to the destruction of the ozone layer, and there you have a much more viable solution to forcing people to live under the surface of the sea(2).

(1) ...next after smoking, air pollution, viruses, and age

(2) ...or to simply become nocturnal.

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    $\begingroup$ Hiroshima was nothing compared to modern thermonuclear bombs $\endgroup$ Aug 23, 2017 at 13:52
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    $\begingroup$ @AmazingDreams Hiroshima and Nagasaki were pure fission bombs and as such much dirtier per amount of explosive yield than fusion bombs. A fusion bomb carried by an ICBM or in a missile or in a aircraft dropped bomb uses only a comparatively small fission charge to get enough pressure to start the fusion reaction, meaning that the contamination is smaller. But the point I was making is that Hiroshima is essentially "clean" today. You get a higher dose of radiation flying to Hiroshima than when visiting the city. Fallout can be removed with simple means. $\endgroup$
    – MichaelK
    Aug 23, 2017 at 14:00
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    $\begingroup$ More comparisons: Rome versus Hiroshima and Nagasaki You get a noticeably higher amount of "shine" from simply visiting a place where the ground is more radioactive than the cities where the atomic bombs were dropped, in this case between 500 and 1000% more. So the point is: nuclear fallout is a very poor plot driver to make people move down into the sea. $\endgroup$
    – MichaelK
    Aug 23, 2017 at 14:03
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    $\begingroup$ @AmazingDreams Well the thing is: if they cannot live on the surface — supposedly because everything is destroyed — then they cannot live under water either. Under water is a very unnatural environment for mankind, and as such underwater facilities require lots of support and maintenance with stuff you can only manufacture on the surface. Imagine your average home aquarium... how long will that last if you stop maintaining it? This would be the same, only reversed. So — again — the nuclear option is a poor plot driver here. $\endgroup$
    – MichaelK
    Aug 23, 2017 at 14:21
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    $\begingroup$ If I apply your logic to the UV solution it would be better to live in caves underground. OP can handwave issues with living underwater if OP wants. OP asked what would happen to the oceans, not if it would be realistic or easy to live underwater. $\endgroup$ Aug 23, 2017 at 14:27

Nuclear fallout will first and foremost affect ocean plankton. Plankton is a mixture: it is part animal (including the larvae of many fish and crustaceans), part plant (microscopic algae). Plankton is the main food source for many creatures in the ocean. Contaminated plankton will kill the animals that eat it, and these animals will, like plankton that dies from other causes including radioactivity itself, sink slowly to the bottom of the ocean. In time it will be replenished, but some will not make it.

Your human survivors will have to eat some contaminated animals, maybe trying to screen out the most contaminated specimens. If they have some mastery of genetic engineering, they could create some enhanced organisms that consume and concentrate the most radioactive parts of the fallout (just as some plants have been naturally found to absorb and concentrate certain heavy metals from the soil). Then they could use those to clean up their underwater fish farms or whatever.

Water itself will disolve some of the radioactive elements and compouns found in the fallout, so water itself will have to be filtered, but if you're living underwater you are doing that already anyway.

One major problem is most life in the oceans is concentrated near the continental shores, which is (not coincidentally) where most minerals and nutrients are washed up by erosion from the continents. That would not be a good place to be. Your survivors would probably do better near deep hydrothermal vents, where they'll have a chance of getting some warmth (and therefore energy) and they'll be farthest from the most damaged areas.

Mutations will surely occur and most will of course have negative effects. In time the flora and fauna might even adapt to radioactivity, but not before lots of species have died out. But mutations, even if they kill people, will not by themselves imperil the future of the human race if they're slow to show their effects. Even if everybody gets a cancer and dies around forty, that's time enough for any healthy couple to raise twenty children (supposing they have any food left to feed them...).

  • $\begingroup$ Since fallout dangerousity is proportional to the atomic weight - lighter elements are less probable to become radiactive, and are less dangerous than heavy metals when do so - wouldn't most of the most radiactive dust just sink to the ocean's bottom? $\endgroup$
    – Rekesoft
    Aug 23, 2017 at 11:42
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    $\begingroup$ @Rekesoft Lighter elements can be radioactive. Maybe you're confusing radioactivity with fissibility. Things denser than water may not sink immediately, and if taken up by organisms, they won't sink until the organism dies. Radioactive elements and compounds may also be water-soluble (NaCl is heavier than water!). $\endgroup$
    – pablodf76
    Aug 23, 2017 at 12:01
  • $\begingroup$ I know that light elements can be radiactive, but the radiation from a nuclear blast has much more chances to turn iron or cobalt to a radiactive isotope than to convert hydrogen atoms into tritium, for example. And radiactive cobalt is a much more nasty stuff than radiactive hydrogen or lithium - lithium isotopes doesn't last a second. However, your point about floatability, solubility and biological absortion are spot on (+1). $\endgroup$
    – Rekesoft
    Aug 23, 2017 at 12:27
  • $\begingroup$ There’s time for a healthy woman to give birth twenty times, but that’s not the same as saying the same couple has time to raise 20 kids, let alone work to provide for them! $\endgroup$
    – jvriesem
    Jan 30, 2019 at 16:57

There is one important point to realize: Fallout is distributed over ground surface, but diluted in ocean volume. That difference is huge. It means, that the same nuclear material that settles in a thin layer on the ground is spread across a few meters of depth at first, and will keep reducing in density as surface water gets mixed with water from deeper layers.

Of course, depending on the intensity of the initial fallout, life in the upper layers of the ocean will be affected. Also, you will see significant impact around river mouths, as those rivers will wash the contamination from the ground to the sea over time. However, you have really good chances for a large amount of sea life to survive even when ground based life is completely eradicated.

Another effect to consider is distribution of fallout: If the fallout is man-made, the vast majority of it will be emitted over ground. There are just no cities to bomb in the middle of the pacific. As such, the fallout will impact the open oceans much less than the costs around highly populated continents.

Taking this into consideration, I guess a good location for your under-water station would be a spot like Hawaii, which is very far from all continents. You don't need many nukes to eradicate life on Hawaii as it's small. However, it is surrounded by a vast ocean across which the initial fallout can distribute and dilute. And the isle is a believable reason for an under-water station to be built on its base. Of course, you can take any other isle either on a hotspot, or on a mid-ocean ridge as well, as long as it's far enough from the next continent.

Bonus points, if you can find an island that is located in a region where ocean water rises from depth to the surface: That will make sure that the station will remain relatively unaffected by the ration for a very long time. And when the fallout finally arrives with the upwelling water many years in the future, it will be diluted real good.

  • $\begingroup$ like hawaii maybe, but not hawaii, the fact that they have a large us naval base pretty much means they are a direct target. perhaps the Galapagos instead $\endgroup$
    – jk.
    Aug 23, 2017 at 11:00
  • $\begingroup$ @jk. I assumed that any bit of land surface was a target... $\endgroup$ Aug 23, 2017 at 11:12
  • $\begingroup$ I don't think there are enough nukes to target every bit of land - the whole world would be effected but not all of it is going to be at ground 0 $\endgroup$
    – jk.
    Aug 23, 2017 at 14:54
  • $\begingroup$ @jk. There are not enough nukes yet. </cassandra mode> But honestly, to get fallout levels so high that all life on land dies from radiation, you'd need to bomb pretty much every single bit of land surface. I mean, there's still life in Hiroshima, Nagasaki, Tschernobyl, Fukushima and all the other places that we managed to contaminate. Life suffers in these places, but it's nowhere near extinct. $\endgroup$ Aug 23, 2017 at 15:06
  • $\begingroup$ my point was that bombing is unlikely to be evenly distributed over the land surface, it will be targeted at military, industrial and population centers, so your first objective is presumably to get away from those centers? $\endgroup$
    – jk.
    Aug 23, 2017 at 15:20

Depending on the scale of the nuclear fallout on your story, the answer might range from totally harmless to fatal. Since your humans are wiped out after the fallout (presumably short time after the explosion), I believe it is a large scale explosion comparable to Hiroshima and Nagasaki nukes, even greater, because it affects global population, so the most probable answer is "fatal". To the ocean in the whole world, too.

Flora and Fauna

The high intensity of radiation from the fallout entering the ocean will "instantly" kill organisms it come to contact with in a few days. The few survivors will have the contaminants enter their system and produce mutations. The first affected will be the plants, then the fish eating the plants, then the fish eating the other fishes.

Can human eat them? Sure, without immediate effects. But after a few years, you will share the same fate with the poor sea life, mutation, that will lead to cancer.

With the scale of the nuclear we are talking about, the result will be likely cataclysmic to the ecosystem, and will likely result on the whole system failing, rather than just worrying about edibility.

Note that your poor outpost will most likely to suffer from not being able to replenish resources from the surface base (fuels, reserved foods, tools, etc.) because of the scale of the destruction and the resulting hostile environment, rather than dying from the radiation, either directly or indirectly.

  • $\begingroup$ The Hiroshima and Nagasaki nukes were tiny: A mere 15 kilotons (Hiroshima) and 21 kilotons (Nagasaki). Compare that to the big cold-war nukes that topped at 50 megatons (Tsar Bomba). $\endgroup$ Aug 22, 2017 at 22:57
  • $\begingroup$ @cmaster :Cold war nukes were hydrogen bombs which work through fusion. They would release a lot more energy, but there would not be a three order of magnitude increase in the amount of radioactive fallout. $\endgroup$ Aug 23, 2017 at 11:40
  • $\begingroup$ @JamesHollis True, the Tsar Bomba was the "cleanest" nuke in history when put in relation to the energy it released. Nevertheless, saying "it is a large scale explosion comparable to Hiroshima and Nagasaki nukes" feels a bit ... weird considering the difference in weapon power between those two first nukes and the ones that would be used to destroy humanity. $\endgroup$ Aug 23, 2017 at 12:10

As always much depends on details, but, in general, rain falls (also) over continents and most of that water returns to the oceans, sooner or later, transporting a lot of material it found on its way.

Any water-soluble radioactive material is bound to end-up in the oceans, sooner or later.

It will be much more diluted than on places of direct impact though.

You will have worst effects at the mouth of certain rivers, before further dilution.

"How much impact" strongly depends on how much radiation, of which kind, and geographic distribution of what is released, but, since you say all ground above sea level is uninhabitable, "prognosis is not good".


You didn't specify what the nuclear fallout is from, so I'm going to assume it is from nuclear bombs instead of reactor meltdowns.

Water is very good at shielding radiation (I know that isn't a great source, but it illustrates the point). It is unlikely that life under the water will experience very much direct harm from airborne nuclear fallout in the short term. Particles falling into the ocean would be more problematic, though very spread out. The ocean is big, and the global nuclear armament is about 14000 bombs, the heaviest of which have 110 lbs of nuclear material. That sounds like a lot, but it really isn't compared to the vastness of the ocean. The main problems would appear in the big predators consuming the lion's share through biomagnification. However, wildlife seems to actually do fairly well with heavy radiation, and would probably adapt pretty quickly by becoming more resistant to mutations and cancer, but mostly would enjoy a long and productive hiatus from human interference. It is unlikely that the radiation could cause any crazy mutations (like speciation). Usually it just causes cancer and shortens life-expectancy.

The radioactive fallout is probably not what wiped out the people though. It is likely a combination of the destructive blasts, a global firestorm, lost infrastructure, and the quickly ensuing nuclear winter. The nuclear winter is likely the thing that will hurt your water-dwelling scientists and other marine life the most. Surface temperatures would quickly plummet, followed gradually by the oceans. The more temperature-sensitive systems like coral and phytoplankton would collapse first, followed by the creatures that rely on them. Deep sea creatures may be affected by the build-up of ash on the sea-floor, but I suspect they would be more resilient than the surface life. Some low-light, cold-loving life like mesozooplankton, ice algae, jellyfish, and other arctic marine life would probably have a population boom as their habitat is temporarily expanded to cover a much wider percentage of the ocean. This would cause an extremely unstable ecosystem as one species after another booms and crashes, possibly on a cycle until temperatures and everything else normalize.


Radioactive material

It depends in the most common radiactive material used during the conflict. This material could maintaint his radiation level for a few seconds and then dissipate totally or million of years still keep emanating radiation. Since this radiation is in the air, raining would bring it to the ocean but this process could increase decay even more over the level of radiation that they can emit.

Here there is an explanation of how the Fukushima leak is affecting the Pacific. http://www.whoi.edu/page.do?pid=127297

Effect of Radiation

Poison for radiation is constant at every moment and from many sources, actually you could get radiation poison just for eating alot of bananas (well...millions of bananas).

The ammount of radiation needed to affect inmediatly a body is really hard to achieve and in most cases you will see only long effect symptoms (like cancer and tumors).

Ocean Depth

Most creatures living at the bottom of the ocean wouldn't even notice a change, probably less food for the reduction of species near the surface or a small change in the temperature of the water. But in most cases unaffected for the amount of water between them and the surface.


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