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Assume humans have the technology and motivation to genetically engineer humans capable of breathing underwater and on land. Also assume for this example mechanical technology is somewhat constrained to make the genetic options more favorable.

Would it be possible to engineer humans that could survive relatively unassisted on land and at extreme depths?

By this I'm referring up to 1km underwater though to depths of 10km (Marina trench) or even further down to 20-30km depths.

I'm aiming to design a society were a mixture of these modified and regular humans would live in an underwater biosphere and would go out to work together in teams of submarines/free divers.

I see four main problems with this.

  1. Pressure at great depths. From my understanding that pressure would crush anything with air in it so regular lungs are out. A gillung as proposed in this question would probably be needed but I'm not sure what other changes humans would need to survive at deep depths.

  2. Lack of oxygen. A human breathing with a modified gill system won't be taking in as much oxygen as an air breathing human. This might cause issues if attempting to work extended periods on low oxygen.

  3. Cold. Deep depths are rather cold but don't seem outside of the range of human survivability. Bring some winter clothes, possibly have a heated section on subs for mermen to warm up.

  4. Pressurizing time. For the mermen to move between the land and sea environment they would need to go through some sort of decompression chamber. If this process is too long/expensive/difficult it could cause problems for the society mixing between the two species.

Trying to find creative solutions to these problems or to find other problems I haven't thought of.

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You do not need genetically engineered humans, normal humans suffice.

Pressure is apart from the lungs not a problem for humans because our body is filled with fluids which are only very slightly compressible and pressure does not influence proper functions. Our bones are solid, but as the pressure comes from all sides, they also don't break or are impaired.

So fill the lungs with a breathable fluid and humans can withstand extreme pressures.

While the movie The Abyss portrayed the idea, many people think it is science-fiction. It is not, breathable fluids like perfluorocarbon are existing and can be really used for this purpose.

So, what you need is an deep-diving apparatus.

  • The human sets up the gear. He breathes or gets an injection which makes him unconscious (You could do this consciously, but it would be torture). Then the gear fills slowly the lungs with a breathable fluid and the gear inserts foldable tubes into the trachea/lungs.

  • Those tubes act as amplifier for breathing. The problem is that a human cannot breathe deep enough to circulate the fluid, so the gear register your breathing and the tubes amplify the breathing to mix the fluid accordingly and replenish it with oxygen. Breathing is therefore only limited by the amount of oxygen. With a nuclear battery and therefore essential limitless energy, the necessary oxygen could be produced by seawater and allows unlimited access.

  • After diving, the human gets unconscious again, he is laid with the head down on a stretcher to pump out the liquid, the gear pumps the fluid outside, collapses and retracts itself and can be taken out. Task finished.

Some remarks to the criticism by Arno Germond: All those points with the exception of High Pressure Nervous Syndrome (HPNS) has nothing to do with liquid breathing, only with the normal gaseous breathing and are therefore literally pointless. To allow breathing, the gases must have the same pressure as the outer water and because oxygen gets toxic with too much partial pressure, the only necessary component oxygen needs to be replaced with nitrogen, hydrogen or helium. This gases are dissolving in the bloodstream/body fluids and are causing unpleasant phenomena like decompression sickness when it bubbles out under lower pressure.

These phenoma do not appear in fluid breathing because there are no other gases than oxygen needed. HPNS is triggered by pressure, but can be counteracted with narcotic gases (you can either use hydrogen as second component or invent a new stabilizing substance which adapts the nerves to high pressure). If the diver emerges, there are no other toxic gases in the bloodstream and therefore no decompression effects.

Middle ear and other air containers (Paranasal sinuses) must be also filled with liquid via Eustachian Tube and sinus openings. Perfluorocarbon is non-toxic and the middle ear is able to remove liquids from the ear. In that case no problems with breaking eardrums are encountered.

I am not a doctor, but I am not aware of "accumulation of toxins" or "high pressure inhibits proper cell functioning". The cells are not shrinking (volume reduction is almost negligible for fluids and solids) and high pressure does not change the reaction rate for solids and liquids (!).

Granted, there will be likely unknown negative effects which will happen in depths of over 1 km in reality, but as long as they are unknown you can ignore them for the sake of the story you are writing.

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  • $\begingroup$ Still have the decompression issue though. $\endgroup$ – John Jan 7 '17 at 7:13
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    $\begingroup$ @John Short answer: no. I have adjusted the answer. $\endgroup$ – Thorsten S. Jan 7 '17 at 19:00
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Look to marine mammals not fish

Seals and whales have been known to dive to 2-3km below the surface. The adaptations you need are not gills (which most likely simply won't work for a mammal) but physiological modifications allowing more oxygen storage in the blood and muscle, more efficient oxygen removal from air, and changing pressure without resulting in the bends. Some whales can dive for over 2 hours but this seems unlikely for humans, perhaps 30-45 minutes would be the maximum. Under your scenario, these modified humans could take extra air while under the water to extend their dives.

It seems likely that these physiological modifications could be extended to allow survival down to 4 or 5kms below the surface but deeper than that might prove impossible.

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  • $\begingroup$ ISTR whales have collapsible/ self reinflating lungs. Unmodified people don't. In either case are the bends a problem if the only gas supply at depth is what was in the lungs when you dived? (Breathe out deeply first, to reach neutral bouyancy). $\endgroup$ – nigel222 Jan 7 '17 at 11:14
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Maybe you could genetically engineer humans to be hybrids of one of these animals, both of which were found living down in the Marianas Trench:

I'm not sure how else you could do it without huge amounts of handwaving.

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Since you are talking about 10km-30km depths, I must disagree with the answer of Thorsten,

Max dive was recorded around 500-600m. High depth dives are associated with gaz toxicity (nitrogen, oxygen, ...), as mentioned, but also: - psychological syndromes, such as High Pressure Nervous Syndrome (HPNS) and the fact you live in the dark - Accumulation of toxins in Blood (your blood & enzymes may not circulate properly leading to accumulation of toxins) - Eardrum breaking with high pressure etc...

For the oxygen issue: well, With helium in the mix, divers can theoretically go down to about 300 meters which can theoretically allow dive of 2-4km, also it has never been done(because most likely a one way dive). But at deeper depth you may expect it also become toxic. So a SF alternative like fluid within lungs, proposed above, to avoid this toxicity issue, why not.

However, you still need to find a way to cancel the issue of the high pressure. Either by cheating (imagine close environment resilient to pressure), or give up on our body and the blood circulatory system (at deep pressure you can expect the cells to not function properly). In fact, creatures living in high-depth have a sponge-like body.

I found similar questions on the net you may find some elements: https://www.quora.com/What-is-the-maximum-depth-a-human-body-can-go-to-under-water-beyond-which-the-pressure-would-be-intolerable

https://www.quora.com/Approximately-how-deep-can-a-human-being-sink-in-water-before-the-pressure-crushes-him-or-her-to-death

https://www.quora.com/Fish-can-tolerate-the-tremendous-pressure-under-water-but-humans-cant-Why

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  • $\begingroup$ Good to mention that really high pressure affects biochemical fundamentals and chemical equilibria.. Life that thrives in that hadean deeps will not thrive on the surface, however much care one takes with depressurization and amounts of dissolved gasses in the water. So people engineered to live at such depths could never visit the surface except inside a pressure vessel. $\endgroup$ – nigel222 Jan 7 '17 at 11:02
  • $\begingroup$ You are claiming that you disagree with my answer and then simply state after enumerating effects of normal gaseous diving at the end "fluid within lungs to avoid this toxicity issue, why not". So you disagree, but Thorsten could be right ? The question itself demands "science-based" not "hard-science", but the answer itself is already hard science and we really do not know the limits if fluid breathing is implemented. $\endgroup$ – Thorsten S. Jan 7 '17 at 19:08
  • $\begingroup$ Sorry Thorsten, I did not mean to offence you, I thought that your reply was based from the perspective that someone who thrive on the surface could go and adapt to deeeeep environements, which I think it is not the case, due to so much hazards (pressure on tissues, cell biology, etc.) as I stipulated in my answer, that is all. That being said, I admitted in my reply that the fluid in lungs and ears etc. may avoid the issue of toxicity. $\endgroup$ – Arno Germond Jan 12 '17 at 8:34

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