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?