I see no problem building a sub surface living space. Seems like a relatively economic thing to do, as well as to be able to hide away quietly from a mad world.

One thing the Ukraine situation has taught is there is little hiding from relatively cheap light aeroplanes, remote controlled, camera equipped "drone" quad-copters, aircraft and the like. With the various GPS systems, there is little protection against simple mortars, bombs, grenades, rockets and cannons (howitzers).

Then there are storms, tornado's, freak weather events, Earth Quakes, solar ejections, potential strange pole-shift events, mad gunman, hungry hoards, wild beasts, nuclear winter, zombie apocalypse.

Having a bolt-hole gives time for a situation to be assessed in relative peace, comfort and safety, whether it's worth coming out at all and attempting to survive. ,