Why wouldnt the idea of digging 8km from the lowest points on Mars work? If we had a fission reactor powered drill that could drill a hole about the same depth (10km but we only need 8km and gravity is 1/3 of Earth on Mars) as the Kola borehole but with a radius of say 2 meters, couldn’t we drop a Tzar Bomb to create an initial cavity and then drop a second Tzar bomb to clear away the side walls to make several square km of surface with Mount Everest equivelant air preassure of 30,500 Pa?
Keep in mind Tzar bombs are mostly hydrogen bomb so not too much of a radio active fall out to deal with.
Then we won’t ever have to worry about decomression and our habs won’t need large submarine style steel air locks(maybe just refrigerator style doors with rubber seal), we could grow plants and trees in plastic green house and have swimming pools in there even. Then we can bulldoze (fission reactor powered) to create more and more of these areas and create a “river of life” out of the new “crater of life”. Why woulnd’t this be the most efficient way to terraform Mars? We could pick areas near the poles where we can pipe water using a fission reactor that melts the ice in the nearest Martian Pole and pipe them to the crater as steam. The water could be used as drilling lubricant and then after the two nukes are dropped, act as water supply for drinking water and making a lake filled with algae to encourage photosynthesis. The lake wont evaporate due to higher air pressures down there.
Also we could plant a fission reactor in the new lake to keep it warm by using the lake as a radiative hydraulic coolant to regulate air temperature by warming air around the lake in the crater and encourage algae growth and allow people to swim in the lake, maybe even grow fish. This area might terraform Mars slowly if we grow it gradually.