I would like to build a colony deep in a mountain - safe from bombardment.
It would be nice if there was some indication when you were deep underground: Deep enough not to be bombarded. Deep enough to benefit from natural cooling, etc.
So far, only one problem: random electrical fires. Within seconds my underground rooms are over 200 degrees. Be nice to invent circuit breakers...
There's already a display toggle in the bottom right that shows roofs. Any tile with overhead mountain (the dark green) is mortar-proof.
Just keep in mind that this will leave you wide open to ant infestation popping out of every corner of your base. :-[
If you're looking for circuit breakers I'd suggest the modding workshop - I've only just gotten back into Rimworld so I'm afraid I can't recall any particular mod other than BT Fuses I think?
But yes, underground is fun. Pop a Dubs nuclear reactor in for fun and you never need to worry about leaving ever again.
Safe our ship2 mod got a nice giant battery with integrated fuse.
With that battery the Short cirquit are harmless.
Sure it is a highend research project, but with some luck you can buy it from caravan/ship.
Or with the Mod RequestItems you might can order it from othe faction, but it isn't cheap.
Dub's hygiene mod got water sprinkle, an alternative to firefoam poppers, that can extinquish fire too.
The mod you are looking for is "RTFuse". Sorry, too lazy to link it, but it's easy to find (and updated for 1.1).
Firefoam poppers are amazing and I get them online as soon as is practical. They shut down fires in an astonishingly wide area with no colonist involvement.