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I was thinking there might be a guest room for humans down in Atlantis. But this raises a question: how would they get enough oxygen for their guests?

Mines have various techniques for oxygenating themselves, including the use of fire, and horse-powered fans (sometimes the two together). So perhaps this could draw air down through a sort of chimney? That chimney would have to be pretty tall, though, and that would risk leaks or other damage.

It would be possible to collect air in water-proof containers, then pull it down to the city, then release the air. That would be quite a lot of work, but it's at least simple.

Does anyone have better ideas for how to keep landlubber envoys from suffocating?

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  • $\begingroup$ What is the tech level? $\endgroup$ – John Dec 25 '20 at 5:08
  • $\begingroup$ @John A little hard to say, since they're a submarinal species. I was hoping for a low-tech answer, for that reason, with technology possible prior to the 19th century. People are welcome to share their thoughts on how later mermaid civilizations would do it, if they'd like. $\endgroup$ – Johnny Dec 25 '20 at 5:51
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Plants. They would need a lot of them, but the plants would remove the carbon dioxide and supply the oxygen, given enough appropriate water and light.

There would also need to be a process to get people back up if the city is very deep, owing to the bends.

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  • $\begingroup$ The difficulty would be in getting sunlight deep underwater. That, or I'd need to have some artificial lighting, either electrical or magical, to feed the plants. It is a good point to raise, all the same. $\endgroup$ – Johnny Dec 24 '20 at 7:23
  • $\begingroup$ It could also be used to raise food for the mermaids. Whether aquatic or land, plants need light, and if they don't have light, they had to import all that, too. $\endgroup$ – Mary Dec 25 '20 at 1:13
  • $\begingroup$ True that they could put the guest house near the farms. They could still be about 250 feet down. Some plants can survive off LEDs, so maybe there's some creature that emits similar light...? $\endgroup$ – Johnny Dec 25 '20 at 2:53
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The mermaids could split water molecules using electrolysis. It's a relatively simple process and it's the same way that astronauts make oxygen. However, the mermaids would have to split a lot of water molecules to supply an entire hotel.

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    $\begingroup$ True. you would need to electrolyse almost 900ml of water per day, per person. That's 3 cups of water. I hope they don't run out of water! $\endgroup$ – PcMan Dec 24 '20 at 6:22
  • $\begingroup$ Hey again @PcMan . The bigger concern is whether they can electrolyse that much water prior to the 20th century. Maybe they have giant electric eels or something...? I don't know how much electricity it takes per ml of water to electrolyse. $\endgroup$ – Johnny Dec 24 '20 at 6:52
  • $\begingroup$ @Johnny you run into these problems with all solutions. Where do you get the power to pump down oxygen in large quantities? Air in water is quite strong. How do you release oxygen in the right amounts and refresh your chamber over time? Don't your mermaids need surface oxygen, as they require lungs to talk? $\endgroup$ – Trioxidane Dec 24 '20 at 10:15
  • $\begingroup$ This solution kills 2 birds with one stone. Electrolysis yields the O2 and the H2 in separate vessels. Pressurized nitrogen causes nitrogen nacrosis. Hydrogen would be a safe substitute. Your visitors would be breathing mostly pressurized hydrogen. Only a little oxygen would be added back. I think 95% H2 and 5% O2 (at pressure so humans would not need full 25% O2) would not be an explosion risk either because of the excess of H2; that is how methane works. $\endgroup$ – Willk Dec 24 '20 at 17:38
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    $\begingroup$ @Willk Bear in mind that increased pressure also increases the UFL threshold. This would also be extremely touchy with regards to any fluctuation in oxygen content, as people will be breathing it. Potentially even having one less person in the room than you're pumping oxygen for would bring the room into flammability threshold. $\endgroup$ – JANXOL Dec 26 '20 at 19:22
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They could use some sort of reverted bucket-wheel excavator, where the bucket-wheel would capture air from above the water and would convey it to an underwater central storage.

bucket-wheel excavator

From there it can be distributed to the local users with a system of piping and pumps, not much different than what happens with gas.

And don't forget that, apart from providing oxygen, they also need to remove the accumulated CO2 from the environments: even though there is plenty of oxygen left in the air, high levels of CO2 are lethal for humans.

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  • $\begingroup$ As a plus, the air could be naturally compressed as you pulled it down. It would take a lot of strength to pull it down, of course. $\endgroup$ – Johnny Dec 24 '20 at 7:10
  • $\begingroup$ Removing CO2 is pretty easy underwater. If the air contacts water (say, by having large parts of the "floor" be open to water, like a diving bell) the CO2 will dissolve and be removed by currents, diffusion, etc. $\endgroup$ – CarlF Dec 24 '20 at 14:27
  • $\begingroup$ The excavator is overkill, they can just have a bucket chain. $\endgroup$ – user253751 Dec 24 '20 at 14:59
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Algae

80% of the earth's free oxygen comes from photosynthesizing algae.

The mermaids could "plant" it in an airtight room, and then place a pipe at the top which goes to a storage reservoir.

Used air would be pumped back into the algae farm to restore the oxygen.

One caveat: they might want to mix some used air into the fresh air; while oxygen is necessary for life, pure oxygen is poisonous.

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    $\begingroup$ This might be the simplest solution. You could have an algae farm at 250 feet, then have a pipe pump the oxygen down lower. The difference in pressure would be interesting.... Poisonous might be a bit much, as the Appolo project used pure oxygen. I'm mostly aware of pure oxygen negatively affecting infants. Though pure oxygen at a high pressure... that's more dangerous, particularly when fire is involved. $\endgroup$ – Johnny Dec 26 '20 at 20:19
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Electrolyze water to produce a mixture of oxygen and hydrogen for breathing.

Purification and other processing may be required to prevent inhalation of halogen gas.

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  • $\begingroup$ Welcome, and thanks for the answer! Electrolysis would be an interesting way of solving the issue. H98 also brought it up in an answer. It was a good point about the halogen gas, though, as he failed to mention that. $\endgroup$ – Johnny Dec 26 '20 at 20:25
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The King of the merpeople has power over the creatures of the sea. He simply commands whales to take a deep breath at the surface and breath out beneath the giant seashell cupola grown for the landpeople's convenience.

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