An interesting theme in KSR's mars trilogy was robot miner machines making a mohole pit and "a couple KM down" the martian atmosphere is either sea level earth

An interesting theme in KSR's mars trilogy was robot miner machines making a mohole pit and "a couple KM down" the martian atmosphere is either sea level earth air pressure or at a somewhat different depth the atmosphere is comfy warm. I don't remember the modeling of how deep or how far apart but it didn't seem terribly unrealistic compared to the grand scheme of teraforming mars. Obviously a thickened atmosphere would not require moholes quite as deep. Possibly to a useful level. True, increasing surface pressure on mars by a factor of ten means you'll still die on the surface, but to a first approximation you just made your mohole 1/10th as deep and 1/10th the construction time and cost. The atmosphere on Mars is actually pretty nice at an altitude of negative a couple kilometers, which doesn't exist naturally but we're quite talented at building horizontal tunnels that long so pointing downward can't be that hard. If you need raw material, robots that never leave the mine don't have scalability problems that human deep mines have and on mars liquid water would be a valuable commodity and robots don't mind water anyway, and solar powered robots can be left alone to generate iron ingots for construction, and for environmental reasons there's no reason to make 100 shallow pits when one deep mine produces just as much and is valuable real estate after digging is complete. On the earth you can't point a large TBM downward because it'll flood or overheat in just a couple miles or less, but on mars it might be quite useful. The problem with moholes is eventually you end up with the submarine problem where no matter how tough your metal hole liner is in an abstract sense, it will collapse under rock pressures. So you can't bore a hole clean thru a planet, however interesting that idea sounds. (edited to add, I looked up data more recent than KSRs old book series, looks like bare rock mines are only possible on mars due to rock blowing out to a depth of 10 KM (better than earth, lower gravity), of course the sky's the limit with steel liners. And at a mere 5 deg/km temp increase on mars, the temp will be cold but more habitable than the surface)