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Deep sea colonization has been dreamt of for nearly a century now but most approaches, such as the recent movie Underwater rely on maintaining the colony at 1ATM rather than circulating mixed gas at ambient pressure. If you don't need to return to the surface, it seems easier to adapt the colonists to the environment. But in my story I need to address the physiological problems such a life poses, in a science-based way. A [different question][1] asks about the maximum depth of "comfortable living" which doesn't ask or provide science-based insights into physiological challenges at a specific depth.

This facility is planned for 540 fathoms depth. Assume the human colonists had ample time to habituate (say, 4 weeks to steadily descend from the surface, maintaining proper partial pressure O2 all the way), and that the He-N2-O2 gas mix in the colony has a ppO2 (partial pressure) of 1.31 bar, and Fraction of Oxygen is 1.3%.

The air will be at ambient pressure - approximately 10,000 kPa. This makes construction vastly cheaper and simpler.

I understand that water would boil somewhere over 500° F and there would be many other physical challenges, but what if any physiological problems could be expected from extended, or even permanent residence in this colony? Please be specific about the physiological issues, using science-based references. Ambiguous answers are not useful answers, so please do not include broad responses such as "they could survive", or "be comfortable" at this depth.

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    $\begingroup$ Does this answer your question? What is the highest pressure humans can live in in comfort? $\endgroup$ – SilverCookies Aug 10 at 7:38
  • $\begingroup$ The other question simply addresses maximum possible depth, not a (relatively focused) list of physiological challenges. $\endgroup$ – elemtilas Aug 10 at 10:07
  • $\begingroup$ Thanks for the similar question @SilverCookies, that question does not ask or help resolve the physiological challenges deep sea habitation poses. For my world it is important to address the specific challenges we would encounter. "Comfort" is not a useful metric for building my world. $\endgroup$ – Vogon Poet Aug 10 at 12:49
  • $\begingroup$ Pretty sure 500 fathoms is closer to 1000 bar than 100 -- and 10,000 kPa is only the lower figure. Nobody tries to breathe heliox at that pressure. $\endgroup$ – Zeiss Ikon Aug 10 at 13:58
  • $\begingroup$ The Wikipedia article on Saturation Diving summarises current knowledge of the subject. The deepest trials have been at 701 metres of seawater equivalent, 384 fathoms, on He-H2-O2 mixtures. So nobody knows for sure, but it seems unlikely that humans can live under such pressures. The partial pressure of O2 in your mix will, in itself, cause oxygen toxicity. $\endgroup$ – John Dallman Aug 10 at 15:51
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Sunlight.

Air and pressure are by far the two biggest considerations for human settlements in the deep ocean, but sunlight is an overlooked contributor to human well-being. As you descend into the deep ocean, there is no sunlight (and therefore no plants) beyond the first few hundred meters. Humans living 540 fathoms down would have no access to sunlight-- or indeed any light at all, except what they bring with them.

This is not an insurmountable problem; you can live perfectly well without sunlight. A cult of 57 people was discovered in 2012 who had lived entirely in an underground bunker; the children in the group had never once seen the sun. The key to survival there is proper diet, so your deep-sea colony will have to pay even closer attention to their diets than surface-dwellers. Since their bodies can't produce vitamin D, they'll need to make sure to get plenty of it from other sources: eggs and fatty fish are common. Sunlight deficiency has been linked to greater risk of various chronic diseases, so your colony will need a steady supply of essential vitamins and minerals.

The sun is also a natural serotonin regulator, without which everyone's circadian rhythms will be off, so sleep problems will be much more common in the deep sea than on the surface. This unfortunately cannot be solved with proper diet, so the persistence of this in certain residents could perhaps be a plot point in your story.

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  • $\begingroup$ Very good points I need to consider, but sunlight can be manufactured indoors. Not a simple task. Maybe a specific arboretum designated for daily relief. $\endgroup$ – Vogon Poet Aug 12 at 21:39
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There is a statutory limit for how long humans can be exposed to that air pressure.

NOAA diving manual states 180 minutes maximum under 1.3 bar partial oxygen pressure.The US Navy has a limit of 60 minutes.

See tables on page 5 and 6 of http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/6310/SPUMS_V26N3_13.pdf?sequence=1

While statutory limits are not physiological limits, they hint that permanent exposure isn't survivable, and, even if it is, will limit the research on adaptability if it is survivable.

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  • $\begingroup$ Welcome to Stack Exchange! I appreciate this and the last part seems relevant - that human research can be limited on earth, however the current statutory limits make the unreasonable assumption that the diver will eventually need to decompress. That does not apply in this scenario. Also, we have sent a chimpanzee to live at 1,000 m depth. We can extend that experiment almost indefinitely to learn our physiological response. $\endgroup$ – Vogon Poet Aug 14 at 12:49

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