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I have an air breathing species that, for reasons of Plot, spend most of their time underwater. They’re pretty much comparable to humans in everything except how long they can hold their breath. Twenty minutes is the average. Only children and the disabled can hold it for less than five minutes.

This particular race live in the submerged, upturned ruins of a past great civilisation. Various air pockets have formed in the ruins submerged in the shallow waters on the coasts of their world which they can use as shelter from the various nasties on the main land masses and also the things that lurk in the deep. These ruins are airtight and pretty much indestructible, so if left undisturbed the air pockets would last a long time.

This race is smart, but pre-industrial, and the air in their homes is obviously finite. Any attempt to equalise the pressure with the surface using hoses etc will flood their homes, so the question is this:

How can this race cycle bad air out of their homes and good air in, using pre-industrial tech

I know some species of spider will capture bubbles of air from the surface and drag them down to their nests to keep a bubble of air for themselves. Could this be a viable tactic for much larger creatures? If the trip to the surface is short enough could the large lung capacity and low oxygen use of this species mean exhaling right after coming from the surface is good enough to refresh the air in their homes for a while?

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    $\begingroup$ Hunt whales and dolphins and steal the air inside of them $\endgroup$
    – user89947
    Sep 6, 2021 at 10:05
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    $\begingroup$ Or farm plankton and algae, they in fact the major producers of oxygen in the world, accounting for 80% of the planets oxygen. $\endgroup$
    – user89947
    Sep 6, 2021 at 10:06
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    $\begingroup$ Exhaling the breath you are holding gives exactly as much oxygen to last as long as you could have held your breath. So if they can hold their breath for 20mins and don't need to "prebreathe" much to raise their blood oxygen content, that's 3 trips per hour or 72 trips to the surface per day. This would also be a great covid spreading environment.... $\endgroup$
    – sdfgeoff
    Sep 6, 2021 at 11:51
  • $\begingroup$ Ideas here? worldbuilding.stackexchange.com/questions/193722/… $\endgroup$
    – Willk
    Sep 6, 2021 at 16:11
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    $\begingroup$ @sdfgeoff: in other words if a ‘trip’ from the surface took 2 minutes one member of this society could provide breathable (and gross) air for about 4 other people (assuming they are constantly swimming up and down)... It’s awful, but might work for a couple of individuals. $\endgroup$
    – Joe Bloggs
    Sep 6, 2021 at 17:20

4 Answers 4

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Long term, big population - they cant without some kind of "ancient artifact"

Short time solutions(for 100 maybe 500 years):

  • leather bags, size around seal, air goes in, stones to make buoyancy lower and You can replace some (100-200liter) air in house/room. Do that 50-100 times per person and You are good for a day(adult human needs around 10 000 - 11 000 litres 20% oxygen air per day). Can transport it in person or can construct something like reversed water wheel with bags instead of cups, work even up to 10m deep.
  • archimedes tube(screw) - can be used to pump some air down under water. Can be used for 2-3 meters deep, maybe 5m if have good and precise enough mechanics.

Small population:

  • plants/fungis/bacteria or so. They generate oxygen slowly, big cave(room) can be replenished with oxygen in month or two. If cycle living in that rooms then can sustain some people
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    $\begingroup$ The Ancient Artifact could be a kind of air conditioning system that The Old Ones built into those ruins, either active and working still for some reason or maybe some kind of passive constructions. It is built to keep the Oxygen levels inside constant and exchanges the CO2 with fresh Oxygen from its surroundings, i.e. the water. Diving Bell Spiders construct the eponymous diving bells under water and due to physics the Oxygen inside is replaced automatically by gas diffusion into the chamber. Nitrogen has to be replaced manually. en.wikipedia.org/wiki/Diving_bell_spider#Diving_bell $\endgroup$
    – Mookuh
    Sep 6, 2021 at 14:26
  • $\begingroup$ I say that the ancient artifact left by the vanished civilization is genetically modified seaweed that pulls air from seawater. As their civilization declined, rising water from polar ice cap melting forced them to find ways to keep living in their now flooded cities. Current low tech just encourages the growth of the seaweed, but they didn't create it. $\endgroup$ Sep 7, 2021 at 16:25
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A Reverse Handpump

Are you aware of what a handpump is? It's a small Mechanical device used to pump underground water up the surface using the concept of the vacuum.

You can create something like a handpump, just the other way round that can be used to pump air from above to below. It is also important to note that it will help in keeping the pressure in your air bubble high if you make changes accordingly.

handpump

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  • $\begingroup$ Rubber or something high-tech needed to operate with presure high enough to go for more than 2 meters deep. $\endgroup$
    – Kamitergh
    Sep 6, 2021 at 11:41
  • $\begingroup$ @Kamitergh That is a very good point. can we not use some type of plant material, weeds, or a tree resin to replace rubber. Or maybe some type of fish or animal bladder/leather. Also pumping air should require less force than pumping water. $\endgroup$
    – V.Aggarwal
    Sep 6, 2021 at 12:17
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    $\begingroup$ @Kamitergh leather was used for this sort of thing, and even hoses. It's a bit less efficient than rubber, but will still work. Leather pistons for bike pumps are still available, for example. This does require surface infrastructure though - you're pushing the air down $\endgroup$
    – Chris H
    Sep 6, 2021 at 13:40
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    $\begingroup$ Latex rubber comes from trees and plants. The only "high tech" part of rubber is vulcanization which adds sulfur and heat to toughen the rubber. But a piston as such is not entirely necessary. You can use a flip top valve or diaphram and use wave motion to pump air. Tubes could be readily taken from plants like bamboo, and latex rubber would make a flexible airtight seal between sections. $\endgroup$
    – Redbud201
    Sep 7, 2021 at 4:25
  • $\begingroup$ @V.Aggarwal not exactly. water have low compresion and high viscosity, air quite high compresion and low viscosity. Tinny gap in water pump is not a problem, can pump high, tiny gap in air pump and almost no presure coming out. $\endgroup$
    – Kamitergh
    Sep 7, 2021 at 7:42
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For reasons of Plot air just bubbles up from underground. Maybe there's some chemical reaction going on down there, maybe it's being pumped down by the civilization who built the ruins, maybe theres some electrochemical process from powered by some still running nuclear plant. It is the mystery of the age: Alchemists have tried to replicate it, budding scientists try to understand it, but no-one knows where it comes from. The religeon mentions a time when the bubbles stopped for several days necessitating extraordianary effort to keep most of the population alive. The prophets warn that one day it will cease altogether, but for now, it is there, so the locals use it.

In Rotorua (one of New Zealands geothermally active regions) it is not uncommon for houses to be heated by tapping steam that comes from underground. It's just "there" and is free energy so why not use it.
In a similar way, your locals have discovered that there is air bubbling up from the seafloor in this one area, and the bubbles get caught by the ruins. As such, thry went "huh, looks like a safe place to hide." They moved there, got good at fishing and din't see a reason to leave.


Air just bubling up solves many problems:

  • Buildings can leak, so long as they leak slow enough.
  • New buildings can be built, old ones can be repaired
  • It gives a reason for them living underwater at that location

But yes, it is "magic"

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From the way you described your scenario, the most simple and effective solution without requiring advanced technology would be a hose or a watertight stone tower that would function as a "air chimney" of sorts. From the way you describe your people its sounds like its possible for them to set up air bubbles next to a site of construction and build up from the ocean floor. You would start by making a stone foundation or by building on top of a solid surface on one of the ruins.

You would build the tower to be a few meters higher than sea level at least, once the tower is finished you can "pump" water out from inside the building into the sea using a Siphon like this: enter image description here

once build, the top of the tower could be manned, you could also have multiple backup towers that funnel air in and out of one or more large buildings on the ocean floor. This could create the need for a corps of volunteers/militia that would function like a sort of fire brigade, except their main duties would be manning operating and repairing the towers, and closing the towers manually using lids incase of unnaturally large waves/emergencies. Although I guess lids would require a sort of sealing agent if that is available.

I hope you find any of this useful.

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  • $\begingroup$ If I get You right then tover need to be at last high as deep is deepest point of "housing dome" to equalize presure in water tubes. For 20m under water housing need to build 20m up over the maximum sea level. $\endgroup$
    – Kamitergh
    Sep 7, 2021 at 8:03

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