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I read somewhere that the lowest depth for a sustainable underwater habitat is estimated to be three-hundred meters. Any lower and the pressure becomes too great for structures that are currently within our means to build.

How would such depths affect a pressurized underwater structure on a planet with lower gravity? Would it be unreasonable to say it could be done and would withstand the sea's erosion over a century or two?

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http://hyperphysics.phy-astr.gsu.edu/hbase/pflu.html#fp

Pstatic fluid = ρgh where
ρ = m/V = fluid density g = acceleration of gravity h = depth of fluid

1/2 acceleration of gravity should allow for double the depth of fluid (IE 300 meters deep on Earth should be the same pressure as 600 meters on a planet with 1/2 the gravity of Earth).

The problem with these underwater structures are the one time events, like an underwater earthquake/tsunami event. A disaster is often a 100% write-off scenario. Maintenance becomes key, as long as the structure is well maintained, 200 years existence is reasonable.

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The most remarkable thing about this expression is what it does not include. The fluid pressure at a given depth does not depend upon the total mass or total volume of the liquid. The above pressure expression is easy to see for the straight, unobstructed column, but not obvious for the cases of different geometry which are shown.

https://en.wikipedia.org/wiki/Hydrostatics#Hydrostatic_pressure for more on Hydrostatic pressure

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  • $\begingroup$ I hadn't thought about earthquakes and the like. Thank you for your input. This facility is meant to be mostly automated, maintenance included. Assuming it's of a strong enough material (and barring any catastrophes) it should stay standing. $\endgroup$ – R. Greenlee Mar 27 '18 at 3:28
  • $\begingroup$ @R.Greenlee - It's a constant problem with underwater structures...day to day seems decent enough, but extend the timeline over a geological time scale and you get these catastrophic event potentials. And it's not simply earthquakes to worry about...rock slides are also an issue bgs.ac.uk/discoveringGeology/hazards/landslides/sea.html Underwater rock formations can be considerably looser and more prone to these events. $\endgroup$ – Twelfth Mar 28 '18 at 22:30
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This makes the assumption that you maintain sea level pressure inside the habitat.

If the pressure is increased and the air mix is something like Trimix then the building doesn't require anywhere near the strength.

The only real issue to people living there is descending and ascending like with normal deep dives.

If the pressure inside is the same inside and out, it makes people entering and leaving the habitat much easier as they don't need to decompress.

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  • $\begingroup$ Thank you very much, I appreciate the answer. This place is meant to be a semi-permanent residence, its inhabitants are there to stay. I haven't heard of Trimix, is that only meant as a temporary fix? I'm wondering if someone could survive on that mix of gasses their entire life. $\endgroup$ – R. Greenlee Mar 27 '18 at 3:33
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My two cents:

Contrary to your assumption, it is not pressure that makes life in the deep sea hard (on Earth). It is the lack of penetration of sun light in below a certain depth. Hence, in regions of volcanic activity underwater, we see sea creatures at the lowest depth imaginable.

Hence my answer is No!

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    $\begingroup$ I'm not sure I understand your response. Lack of sunlight will not crush my proposed structure like a soda can (of your choosing of course -- I prefer Dr. Pepper). The facility has been built, despite the warnings of naysayers and skeptics, and can indeed withstand the thirty-plus atmospheres of pressure. From there is it unreasonable that its creators would be capable of heating and lighting the place? $\endgroup$ – R. Greenlee Mar 27 '18 at 6:54
  • $\begingroup$ I assumed you were talking about a natural habitat (that still poses the question: why build it there then?). My bad. The light is needed for the plants to have energy. Then it is not unreasonable to make artificial lighting (or other source of energy) for the plants. I am not sure about the erosion though. $\endgroup$ – Kavi Vaidya Mar 27 '18 at 16:51

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