I think the threat model is packs of desperate people looking to take your stuff, not bombs.

Being underground won't help much if your assailants decide to smoke you out. To stop that you'd need to shoot back or have a very robust and well designed fortification, with air filtration and redundant hidden vents. If you were committed to fighting back, an above ground fortification made out of reinforced concrete would be easier/cheaper to construct and would probably give you a better view of your surroundings. You'll also need several people you trust living there with you, so you could keep lookouts stationed around the clock and cover for each other.

All in all, it seems like a hopeless scenario. A better approach is probably to make friends with your neighbors and try not to look like you have anything worth stealing.

> If you were committed to fighting back, an above ground fortification made out of reinforced concrete would be easier/cheaper to construct and would probably give you a better view of your surroundings. You'll also need several people you trust living there with you, so you could keep lookouts stationed around the clock and cover for each other.

A castle. You're describing a castle. :)

Invest in bunker REITs in Eastern Poland![1] Either way bunkers tend to be blown up by one side or another...

[1] - https://en.wikipedia.org/wiki/Wolf%27s_Lair

> try not to look like you have anything worth stealing

I think you said it all :)

If you're only building a small "bunker", it's not that expensive to build down there. Just a backhoe, a few hours, 6 sides of concrete and a tin roof.