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This is strictly Earth based. Would it be possible for a society to develop their own civilization underwater (nothing fantastical like Aquaman or whatever, but more of a grimy, hardscrabble existence where they've developed a food chain from geothermal vents, and physiological adaptations such as biolumination, reduced vision but ability to echolocate (just brainstorming).

What reason could this society have to come up to the surface to a post-apocalyptic Earth that's been ravaged by nuclear war and bioweapons?

Perhaps they're escaping from an ancient Lovecraftian entity that's been hibernating for eons, and the explosions from the bombs awakened it? Now the underwater denizens are escaping from it?

Or maybe their food sources are running out due to widespread extinctions in the overall ocean food chain, and they're forced to come up to the surface and try their luck there, which creates a conflict with the humans living aboveground? Essentially, the nuclear warfare led to the food chain and wildlife getting screwed up, so in a nice little bit of irony, it's the surface-dweller's fault that the ocean-dwellers are coming after their land.

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  • $\begingroup$ Is the surface still unlivable, or it's more or less back to normal? If it is the latter, we can have various "recolonization" scenarios (ex: "WALL-E"). $\endgroup$ – Alexander Sep 20 '19 at 16:38
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The decay and detritus of the surface dwellers is beginning to impact their society.

Leaking nuclear waste, mass mutating bioweapons, ash, all these things are threatening their own way of life. So they send expeditions up to the surface to explore our ruins, fix our leaking waste and scrub radiation, destroy our bioweapons.

This would only really need small scale surface missions and activity however as they would be trying to fix our mess. If you need mass movement, I think the ideas in your final paragraph are very good.

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A deep ocean society relies upon geothermal energy to live. What if, for reasons that don't have to be explained anytime soon in the story, the geothermal energy subsides to nothing at the ocean floor? Without energy, the community becomes unsustainable and has to go to the surface to get the energy needed to make food and sustain civilization. So, they migrate to the ruined land.

Perhaps the real cause of the declining geothermal energy is along the lines of the anime "Darling in the Franxx" in which some other civilization, local or alien or arisen post-apocalyptically, has started to tap into and suck up geothermal energy from the Earth that is depleting natural sources.

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What reason would a hidden, deep ocean society have to come up to the surface of a post-apocalyptic Earth and claim the land for themselves?

I can think of only two offhand... revenge or reclamation.

Revenge... they don't intend to claim the surface for themselves, after all, it is: horrible, hot, dry, the pressure is all wrong, the sounds are all wrong, the light is blinding and the sun is burning, the food is weird... the list goes on forever. The surface is such an utterly alien environment that they're just not interested in it, just interested in getting rid of the stupid primates who live up there to make sure that they don't get the opportunity to do the same stupid awful things all over again.

They don't want to be there, they're not going to stay there for long, and the sooner they can get back down to the deeps the better. They're angry at you personally, for ruining everything and making them come up to this arid hellscape, and they're going to make you pay.

If you've not already done so, there's relevant reading in The Kraken Wakes or The Swarm (the one by Frank Schätzing) or Peter Watts' Rifters series (available as free ebooks from the author's site).

Reclamation... they used to live on land, or perhaps on the coast, but were driven away by human expansion, suffered badly from disease and pollution, gained a sort of mythical status rather than being taken seriously by the bulk of humanity and were ultimately scoured from the surface. Or maybe they were human, modified by some technological process or experiencing some kind of (at least partially) benign mutation, and were able to retreat to the oceans when the surface humans rejected them. Previous attempts to return failed, but not this one. This time the humans have been weakened beyond the ability to effectively fight back.

They may have adapted to their new undersea homes, but they came from the surface before and they've always wanted to return. Re-adapting to the surface world will be difficult, but they're biologically equipped for it and it'll come back to them in due course.

Perhaps they're escaping from an ancient Lovecraftian entity [...] Or maybe their food sources are running out due to widespread extinctions in the overall ocean food chain, and they're forced to come up to the surface and try their luck there,

Probably not so much... The ocean is huge, and the surface is utterly alien and extremely hostile to native undersea species. They'd move into shallower seas or coastal waters, at most. But to move from the abyssal plains to the surface? About as likely as you wanting to make the opposite journey and live in the dark and eat weird shellfish for the rest of your life.

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Our old ruined stuff is still pretty sweet.

Your submariners have cargo cults.

https://en.wikipedia.org/wiki/Cargo_cult

Cargo cults are marked by a number of common characteristics, including a "myth-dream" that is a synthesis of indigenous and foreign elements; the expectation of help from the ancestors; charismatic leaders; and lastly, belief in the appearance of an abundance of goods.[7] The indigenous societies of Melanesia were typically characterized by a "big man" political system in which individuals gained prestige through gift exchanges. The more wealth a man could distribute, the more people in his debt, and the greater his renown. Those who were unable to reciprocate were identified as "rubbish men"....

Manufactured goods from the surface have always been valued in the deep ocean - things like plastics and refined metals have no indigenous equivalents. They are worth a lot and make excellent gifts. With the fall of civilization, vast troves of these things are now undefended. The submariners foray into the ruins to find loot - steel, plastics, glass and the like. These things still exist in abundance, but it is dangerous and scary for the sea people to come onto land and get them. If you succeed and bring back a good haul, it means you gain in wealth and more importantly prestige. It is worth the risk for some.

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    $\begingroup$ Bonus points if the refined metals and plastics are the thing that lets the submariners voyage to the surface. Nothing says ‘desperate’ like a cobbled-together exosuit built from the trash and ruins of a dead civilisation. $\endgroup$ – Joe Bloggs Sep 21 '19 at 19:09
  • $\begingroup$ Also: given the number of container ships that would sink in a nuclear war: literal cargo cult. $\endgroup$ – Joe Bloggs Sep 21 '19 at 19:17
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Well, for Humans mass migrations (and the claiming of already inhibited territories) seemingly mostly were triggered by either the "homeland" becoming un-inhabitable due to climate changes and the shortage of food that comes with it or by aggressors driving the population out of their homeland en masse (maybe because they, in turn, are facing one of those problems in their homeland).

Also, IMHO, the founding of colonies in "foreign lands" by white settlers/colonists might serve as inspiration here. Many looking for a better life (maybe because belong to a minority that's being oppressed in their native home), and too many just thinking they could make a good profit. If you go in this direction, maybe your deep ocean society only just discovered that there's inhabitable land above the surface. Maybe it just was never believed that there was any, maybe it was suspected or known, but unthinkable to go there until the need was strong enough, or there was a major shift in their society.

Or, maybe the land above and its atmosphere in fact just where not hospitable for them before the bio-nuclear Ragnarök.

Just a few thoughts :)

Edit: grammar

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Increases in atmospheric carbon dioxide causes catastrophic increases in carbonic acid in all open bodies of water, the decreased pH causes the pelagic food chain to collapse and a mass extinction event occurs.

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Here's a simple answer: curiosity. After centuries of rising noise and trash-dumping from the surface, the sudden silence might get them reasonably concerned, and start sending scouting parties above.

Question: can your submariners deal comfortably with the changes in light and pressure?

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