Digging 8km under the lowest point on Mars will get us Mount Everest conditions for air pressure.
Nuclear powered bulldozers working around the clock to clear away dirt on a low point on Mars such as https://en.wikipedia.org/wiki/Hellas_Planitia or a place with similar elevation preferably near the poles where there is lots of ice to feed the colony with rocket fuel, water and oxygen and even more CO2 gas. The lower gravity of Mars should allow us to build bigger dozers that has more mass and can accommodate a submarine style reactor.
Drop a hydrogen bomb down an 8km shaft to create a cavity and then clear away the sides to make it accessible as a gradual slope like a crater. Maybe a second bomb to do the clearing of the sides or nuclear powered dozers in option one above.
Would this be a practical means of starting a colony? We have experience with underground nuclear testing and we know that it can create cavities, maybe we don't even need to clear the sides and live in the cavity like a steep tear drop shape: https://en.wikipedia.org/wiki/Under...a/File:Nuclear_explosion_craters_schema_1.png https://en.wikipedia.org/wiki/Underground_nuclear_weapons_testing#:~:text=The extreme heat and pressure,cracked, and irreversibly strained rock.
There's a whole science in making a nuclear crater, why can't we use it on Mars to get air pressure?
Then we can not worry about decompression when we build. We can just carry scuba tanks with Oxygen and Nitrogen and breathe through a regulator.
If we use H bomb instead of pure Fission Bomb there should be less radioactivity problems.