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My story takes place hundreds of years in the future after global warming has made life at the surface uninhabitable for most life forms. What’s left of mankind survived because it became popular to build cities underwater before things at the surface became dire.

Obviously these cities would have to be self sustaining in order for humanity to survive. I envision them growing food in hydroponic pods, raising livestock, using 3D printers, desalination equipment for a water source.

I would like some of these cities to be more advanced than others because they discovered ways to make new infrastructure and technological advances while stuck underwater.

I’m not writing hard core science fiction but I don’t want the infrastructure and technology to sound like complete fantasy. Is it possible for humans to build a completely sustainable city underwater if money was no issue?

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  • $\begingroup$ If the surface is uninhabitably warm, then so is the sea I think. Anyways ... how are the going to power their cities? Nuclear fusion would be possible (the sea has lithium and deuterium in abundance), but that's pretty high-tech! Otherwise perhaps oceanic currents ... $\endgroup$ – Karl Jan 4 at 19:50
  • $\begingroup$ Thank you I forgot to mention that, I was thinking wave turbines. I will have to look more into nuclear fusion for the advanced cities, thanks! $\endgroup$ – HR Allsop Jan 5 at 15:51
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Soan's answer raises some really good questions that deserve answers. I was gonna make this a comment but it got way too long; so, full credit to him for what variables to consider.

The ideal place to build cities would be on the continental shelf within a few kilometers of the shore. While this would not put it in still water, there are many logistical reasons to do it this way. First is that these cities are built by land dwelling people; so, the closer they are to these now defunct infrastructures, the easier they are to build. Secondly, is that if you try to build past the shelf, you would be building on a cliff side of the continental slope which would be much more risky in the case of erosion, and the abyssal plane is way past the crush depth of modern submarines (that have large enough of interiors to support long term life). Apart from being at non-crushing depths, this also ties into the power grid issue because you could build hydro turbine power farms around your city to harness the tidal current of the continental shelf.

For building materials, you could replace steel with aluminum. Unlike iron alloys, when aluminum oxidizes, it becomes stronger; so, structures made out of it would be able to last for centuries under water. There is also the option of using portland cement to make concrete structures that cure underwater. While not as watertight as aluminum, this would solve your erosion issues. By planting your aluminum structures in heavy, porus, concrete foundations, your foundation would become overgrown and built up with coral faster than it erodes. As for structural shapes, while a dome (or sphere) is an ideal shape for underwater construction, you are likely picturing a giant glass dome but that creates a lot of seams and structural issues. The most logical configuration would be a series of hundreds of modular, interconnected, submarine like structures with plain metal or polymer exteriors, concrete bases, and minimal portholes and waterlocks.

While things would grow all over your exterior (bacteria, coral, barnacles, etc.) it would not make for any hernderance to survivability. Over-time, your city may be completely buried by sediment and coral, but things that are often used like water locks for your mining subs or outlets for bilge pumps would be be used often enough that they would stay clear with occasional maintenance

As for temperature, the average depth of the shelf is ~140 m (450ft), but only the top 100m of the ocean are particularly affected by surface whether. At 150m, the temperature does not vary nearly as much; so, where on the land might see an 80deg variance throughout the year, at this depth, your variance would be so much smaller that it may always be a bit warm, but you would not see fatal heat waves like on the surface. Also, photosynthesis is possible at depths of up to 200m, meaning you can farm with sunlight at this depth (though probably using specialized low-light crops or seaweed farming).

If you want to make "advanced" civilizations; you could have them build in the deeper waters in the Abysmal plane. Instead of hydro-turbine power, they could harness geothermal power and mine the exposed bedrock of the continental rise giving them access to vastly more power and resources. Their structures would be similar in shape and layout but be made from much more advanced materials like graphene laminates to be able to endure the greater pressures.

As far as plot hooks, it's pretty easy to see how superstitions and mistrust would build overtime between the high-born people who live in the light of the sun, and the low-born people who live in the total darkness of the abyss.

[edit]

As for industrialization, unlike a space station, these colonies have the option of building "smoke stacks" that lead up to the atmosphere. This means that you could pipe in the Oxygen needed for your smelting equipment. While this air may be toxic and rather hot, it would still be suitable in a closed system for doing all the industrial things you can do with an abundant atmosphere on the surface. This also means when mining or expanding structures, you can just pipe in more air to fill the new spaces after cooling, filtering, and compressing it.

With this in mind, I suspect mining would look one of three ways depending on the resource. One type would involve making a waterlock a little bit away from your city that just leads to a tunnel going down deep enough into the bedrock to make for a mostly airtight seal. Then you would use bilge pumps to clear any water that drips through and fill the tunnel with air conditioned surface air as you expand it. The second type of mining would be for the silicates and diatoms that make the actual sea bed which would probably just be shoveled up by specialed submarines. The third type would be for things that you have to return to the surface for. For this people may need to go back to land in what basically look like EVA suits to mine for things that may only accumulate under non-sea bed conditions or to recycle rare pre-fall materials that they can not manufacture with their limited surviving tech.

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  • $\begingroup$ Totally forgot the tides and the food problem so thanks for expanding on my answer. $\endgroup$ – Soan Jan 4 at 22:15
  • $\begingroup$ the abyssal plan is not past the ability of submarines, it's just of no military interest. civilian submarines designed forthe task have gone down there and returned intact. just military submarines aren't capable. $\endgroup$ – Jasen Jan 5 at 4:45
  • $\begingroup$ Thank you for the answer. I was thinking about the advanced city having mines and tried to research underwater mines but so far haven’t really found much. If it was under a dome structure could they mine for resources and process them without accessing the surface? $\endgroup$ – HR Allsop Jan 5 at 15:49
  • $\begingroup$ @Jasen, it is true that the technology exists, but those subs are basically tiny cockpits surrounded by lots of costly materials, I've updated my answer to reflect that I meant submarines with long term living spaces. $\endgroup$ – Nosajimiki Jan 7 at 17:34
  • $\begingroup$ @HR Allsop, I've added additional feedback with ideas for industrialization. $\endgroup$ – Nosajimiki Jan 7 at 17:35
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Yes it would be possible.

Things to consider though:

  • These cities would need to be built in an area where the water doesn't move much because of erosion.
  • Also most metal constructs would degrade within decades/ bit more than a century like the titanic. Extensive and regular cleaning might keep metal constructs viable for a few centuries. Building with rocks would be another solution.
  • Also micro bacteria would probably infest everything exposed to the ocean.
  • Depending on the heat on the surface temperature your cities would need to be deep within the oceans. So the high pressure needs to be dealt with a dome enclosing the cities could do the trick but the dome would need to be very thick depending on the deepness of your cities. With the dome you could also deal with all of the micro bacteria as it would only infest the dome.
  • Power generation. You could use differences in heat (warm air or water from the surface and cold water from around your city) to create a heat exchanger when your cities are deep enough they could use the heat from the earth crust. They would need to do a bit of drilling to get away from the water cooled part of the crust and into the heated part. You could also use under water currents your cities should still stay away from them for longevity but with a long cable to your cities it should be fine.
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    $\begingroup$ Don't bother with metal, any kind of solid plastic survives practically indefinitely underwater. ... unless it gets too warm. $\endgroup$ – Karl Jan 4 at 21:10
  • $\begingroup$ Most plastics decompose with a few hundred years. Polymers would make for good internal or temporary structures, but would probably decompose enough to risk cave-ins within a single centry if used for major structures. $\endgroup$ – Nosajimiki Jan 4 at 21:50
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Don't try to grow land based plants and animals, the ocean is already very abundant in these.

Windmills and solar panels could still just out above the city, but you could also harness waves.

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  • $\begingroup$ From my research I believe that if the surface was too warm for humans to survive outside (it’s hot enough that they would be overcome with heat exhaustion in a matter of minutes) the ocean would have absorbed enough CO2 to devasted ocean life through acidification. I would imagine there would be some sort of creatures of maybe algae that survived, but in my story most of the sea life has gone extinct as well $\endgroup$ – HR Allsop Jan 5 at 15:22

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