UPDATE:
Oxygen is lighter than CO2 so the tunnel will need to be adjusted accordingly:
Humans can settle in the Oxygen bubble cavities that are above a lower CO2 zone that has a water lake that the nuclear thermal reactor uses as a radiator thus keeping it warm. This CO2 zone can be rich vegetation zone with algae and trees converting CO2 to oxygen that floats into the habitable region accessible by ramp.
ORIGINAL ARTICLE:
Considering the scale height of Mars being 11km and Earth’s 8km, the difference considering the lower Martian gravity gives us hope to live inside rather than on Mars without preassure suits, a 55km deep tunnel on Mars with a “silo” built into the vertical shaft can allow humans to live in Mars in the lower end of the tunnel where there is 1 bar of atmospheric preassure, there and any cavity excavated out horizontally after that.
The silo could be a nuclear thermal powered structure with a built in drill bit. We can have artificial lighting powered by a nuclear thermal reactor and waste heat from it could keep the air warm by heating artificial lakes.
What is a scale height:
https://en.wikipedia.org/wiki/Scale_height
If built at the equator we get sunlight directly under the vertical shaft, if built at the poles we will need a system of mirrors and crystals to light the bottom portion. The poles is more guaranteed to provide us with ice and water that we will need to live on that our thermal reactor can heat and melt.
Inspiration of the megastructure from the TV series SILO: