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Would air pressure be demonstrably greater in a subterranean city at a depth of 1 mile? Lava tubes leading into a dormant magma chamber -- this would have challenging gasses that would need to be ventilated to make it habitable, right? The consensus for the geothermal temperature gradient seems to be about 1-2 degrees F for every 100 feet of depth, so if we're starting our thermometer at the point where ground temperature mimics the average atmospheric temperature (I believe it's about 30 feet down), then I'm thinking it'll be about 75-80F at one mile down.

Air pressure below the surface adds roughly 14.7PSI per 3200feet. So it would be more compressed air, further down, but as far as I can tell, "liveable." I found this article that states 7500psi for every mile you dig:

Geologists calculate that, for every mile you dig beneath the Earth's surface, the temperature rises 15º F and the pressure increases simultaneously at a rate of about 7,300 pounds per square inch.

So... 7300psi of air pressure at 1-mile depth? That doesn't seem right when we have active mines 2.4 miles deep.

The other part I haven't been able to research is whether air quality would be completely uninhabitable at that depth. If you're walking in a lava tube that eventually vents to the surface, how breathable would the air be with respect to pressure/oxygen levels?

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    $\begingroup$ 7300 PSI must be for earth pressure, not air pressure. $\endgroup$
    – void_ptr
    Dec 15, 2020 at 18:59
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    $\begingroup$ The elevation of the surface of the Earth is not equivalent to sea level. An underground facility a mile deep on the Tibetan Plateau would still be over a mile and a half above sea level. $\endgroup$
    – Keith Morrison
    Dec 15, 2020 at 23:12

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Use mines as examples:

1 mile deep (1.6km), is nothing compared to some modern mines. 10 minutes drive from here there is a mine that is 3.6km deep.
We can make such mines livable, or at least tolerable, for longterm workers.

Temperature:
At 1.6km depth, the temperature will be 25C * 1.6 = 40C (72f) above the average surface temperature. Hereabout the surface is about 20C, so it is an unbearably hot 60C (140f) even just 1.6km down.

Or it would be, if not for ventilation. To keep the mine habitable for workers, they are constantly pumping groundwater out, and huge volumes of surface air in. This surface air is acceptably cool, and replaces the extreme heat expected in the mine with a mere sweltering heat. Down from 60C to about 32-35C.

Air pressure:
Pressure is much less of a problem, and only really manifests itself when workers move from the depths to the surface in rapid elevators. Even then it's no worse than ascending on an airliner to cruising altitude.

AT 1.6km, if the entrance is at sealevel, the airpressure down below will be 1.2 atmospheres. About the same pressure as swimming at the bottom of a household pool.

Air quality:
Without forced ventilation you have severe heat problems, as noted above. Even if you were to install an in-place recirculating air cooler, you would have problems. There is no oxygen source down there. If you do not get oxygen from fresh surface air, you will have to supplement it from some other source. In addition you are surrounded by rock. This rock is continuously leaking water into the voids in it, also trace gases including such interesting ones as Radioactive Radon. No problem if you exchange your air all the time, but if you allow it to accumulate over months it becomes a health hazard.

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    $\begingroup$ Thank you, @PcMan, it sounds like I would need to have some cooling water for heat exchange as well as a way to draw air through to keep gases from accumulating. $\endgroup$
    – Ceci B
    Dec 15, 2020 at 21:10
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It would be hot underground. Geothermal gradient indicates that on Earth, 1 mile underground would be about 40-45 C (75-80F, just as you said) hotter than on the surface. Unless your underground city lies under permafrost, that would be a definite challenge for human habitation.

The air pressure would be high (although not unbearably high). One mile deep it's going to be about 1.2 atm

Ventilation would be a challenge too. Underground city needs a lot of fresh air, and rocks around tunnels tend to release various gases, many of them harmful to humans. I don't know if it's realistic to have a livable city that deep without forced ventilation.

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  • $\begingroup$ 40 C is a good estimate, but that corresponds to 104 F $\endgroup$
    – Gary Walker
    Dec 15, 2020 at 20:12
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    $\begingroup$ @GaryWalker That 40C is a 72 F increment over whatever the surface average temperature is. 40C = 72F, don't be fooled by the problem of the two scales not having the same zero baseline. $\endgroup$
    – PcMan
    Dec 15, 2020 at 20:24
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I present the Sudbury Neutrino Observatory (SNO), now SNOLAB, located 2100 meters down in the still operating Creighton Mine (the operating faces are currently down around 2400 meters with exploration to 3000 meters). Sudbury is 350 meters or so above sea level, so SNOLAB is about 1750 meters, or 1.1 miles, below sea level.

You will note the distinct lack of obvious challenges to people working there comfortably, so, you could ask for theoretical calculations based on assumed temperature and pressure gradients. Or just, y'know, look at it.

https://mcdonaldinstitute.ca/wp-content/uploads/2020/07/DSCF1834_pc_ZacKenny-1176x720.jpg

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    $\begingroup$ Shall we ignore such little factiods as "The ambient rock temperature at this depth is 42C", the 1.6Megawatt power allocation to HVAC, the special precautions they need to make because "Radon (222Rn) is of special concern in the SNOdetector, since it can diffuse from components, dust or the cavity walls into... "? There are no obvious challenges to people working there BECAUSE those challenges have all been countered by a lot of heavy machinery and a hefty power bugdet! $\endgroup$
    – PcMan
    Dec 16, 2020 at 10:33
  • $\begingroup$ And? That would be an issue in any event. $\endgroup$
    – Keith Morrison
    Dec 16, 2020 at 11:53
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    $\begingroup$ Which is exactly what the OP is asking: a enumeration and quantification of the issues that do need to be dealt with. No-one is saying they cannot be dealt with, you answer is a fine example of an instance where the environment is well dealt with. There is a "distinct lack of obvious challenges to people working there comfortably" BECAUSE the problems have already been addressed. Unfortunately, your snarky comment of "Or just, y'know, look at it." on this seems to indicate there was never a problem to be handled in the first place. This is untrue, and a bit unfair of you to show it as such. $\endgroup$
    – PcMan
    Dec 16, 2020 at 19:03

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