Not sure what increasing the atmospheric pressure would do given that of course the atmosphere is not even remotely breathable. What is perhaps less well known is that the Martian atmosphere not only lacks appreciable amounts of oxygen, but it also contains toxic levels of carbon monoxide – in fact twice as much CO as the earth has CO2.
You could maybe dig it out using asteroid impacts or something. +++++++++++++++++++++++++++++++++++++++= OK, but how to get the steroidal material out of the hole?
Market it to gym rats and bodybuilders? 
But you wouldn’t have to pull any of the asteroid out of the hole. Craters are much, much larger than the asteroids that create them (and tend to be obliterated by the impact anyway).
OOPS Too late to edit, I meant asteroidal material a bit different than Steroidal!
Well, I suppose one goal of thickening the atmosphere would be to promote global warming and melt water ice, forming some lakes or seas and rivers and things. Then I guess we could add some oxygenating algae. It’d pretty hard to ever get the atmosphere just right though. If we ever colonize Mars, it’s probably going to be mostly indoor living.
Possibly Never - no matter how much you dig, you’ll** never **get to air pressure to be same as Earth at sea level. The Martian “air” will probably turn into dry ice before you reach said depth.
One big thing neglected in this whole thread is the thermodynamic properties of CO2 (Martian atmosphere is 95%+ CO2) and the prevalent temperatures on Mars which is -89 to -31 C ( -128 F to -24 F)..
See the phase diagram of CO2 here. At these temperatures, increasing the pressures by a few hundred millibars even will turn the atmosphere into solid.
The reason, I say possibly is because the underground temperature of Mars increases by5 degrees per km - so that calls for more intricate calculations.
This equation is a gross simplification and does not work for mars and certainly does not work for deep underground even on earth. Here’s why :
(I believe this equation is the same as this Wikipedia equation)
1> If you notice - this assumes that the atmosphere assumes ideal gas laws. This approximates earth’s atmosphere. For the Martian atmosphere, the low temperature and 95%+ CO2 means the gas is extremely non-ideal. You need to use the CO2 equation of state for this calc.
2> The entire atmosphere is taken to be at the same temperature (some mean temperature - Wiki says it is 250K while NASA says it is 288K - go figure). This may work for earth and give some average indication.
3> If you were to drill holes even on earth (10s of km deep), the temperature in the hole will dictate the pressure more than the air column of air sitting above it. Earth temperature increases at 25C per km - so a 10 km hole will have a temperature of about 540F - thereby setting a chimney effect on the hole.
“Asteroidal” material would, by the very composition of that word, not be steroidal.
Examples:
– Prednisone: Steroidal.
– Aspirin, Ibuprofen, acetaminophen (paracetamol), etc.: Asteroidal.
Digging a hole on Mars then filling it with breathable atmosphere might be a useful colonisation strategy, since the amount of atmosphere required would be significantly smaller than would be required to terraform Mars completely. The rim of the canyon, or crater, would need to be high enough off the canyon floor to reduce atmospheric loss over the brim to a minimum.
However it would be easier to make a greenhouse or dome by inflating a transparent membrane over your colony to prevent atmospheric loss; this membrane, if made of the right material, would have the added advantage of keeping the ground surface warm by retaining infra-red radiation, which would not happen in an open pit (nor would it happen on a terraformed Mars).
To keep the ground level warm in an open pit or on a fully terraformed Mars you would need to add greenhouse gases, the most common of which (CO2) would be toxic at the necessary partial pressure.
Perhaps the best strategy is a hybrid one; find a deep canyon or crater, or dig one, then roof it over.
Boreholes are lined by metal casings, which can’t really happen in the deep mines - rockbolts and gunite are the best we ever did at stabilization in the 2km-deep mines I worked in, and then only in the tunnels, not the stopes.
Well of course it’s a simplification - no one is paying me to be an expert in Martian atmospheric modelling especially in situations so far from existing conditions. I mean the OP is trying to estimate how to increase the thinnest of surface pressures by a factor of 166.
NASA presents p = .699 * exp(-0.00009 * h) as a reasonable metric model for lower Martian atmospheric pressure. You are however completely correct, you will wind up with temperature effects once you start digging deep enough, this will impact the scale heights and drag in all sorts of caveats and conditions.
It is just the neatest thing to kick about on a pad of graph paper though.
It’s nice to have a decent atmospheric pressure even if it’s not breathable. Vacuum, or Mars’s current atmosphere, (there’s not much difference) means that you need to walk around in a bulky pressure suit like the Apollo astronauts wore. With a decent atmospheric pressure, you could get by with shirt sleeves and a rebreather mask on your face. Or if you go with a domed habitat, the dome will only need to be strong enough to support its weight, and won’t also need to stand up to the pressure difference between inside and out.
It’s one thing to put casing into a well that’s a mile or two deep. But eight miles? Can they really run casing that deep? More to the point, did they actually do this for the Kola hole?
Don’t know about 8 miles, but this cite says one of its sub-boreholes was cased with steel to 8278m while the borehole is 8578m deep. So at least that deep…
…and I’d be very surprised if the even-longer-but-not-as-deep Al Shaheen oil hole isn’t cased, flowing oil would do nasty things to the sidewall if it wasn’t.
Wait, are you one of those chemists who can’t decide whether it’s better to be unionized or ionized?
A combination approach sounds good: Start in the deepest place you can, dig down some, then roof it over with a pressure dome.
Nah, you just dig a little hole until you find the big cavern where the ancient Martians stored all their atmosphere. Then the big glow rod thingies release that atmosphere and you can just go outside and starting building a house.
Or did I dream all that?..
