NATIONS lived UNDERGROUND in the past. WHY the F would they have done that?

I mean... the more we learn about our surroundings, the more we understand that at some point in history, humankind had underground habitats that could house MILLIONS of people.

They had undergound individual cities, that had several floors, and within a small footprint, even TENS OF THOUSANDS OF PEOPLE have lived.

I mean just a few small links, you'll find better ones yourself duckduckgoing:

[link to www.ancient-origins.net (secure)]

[link to www.tellerreport.com (secure)]

[link to www.theguardian.com (secure)]

[link to curiosmos.com (secure)]


I mean... they are EVERYWHERE in the world. We have cities, human made caves, natural caves, pyramids, tunnels, the works. Even the Giza plateau has miles and miles of underground tunnels, even most of the pyramids have their actual chambers UNDERGROUND, covered by a massive pile of rocks.

WHY?

All "historians" and "archealogists" say that people were hiding from wars and attacks and hot & cold, but that's way too easy. You don't dig an underground city for 30'000 people to hide from invading armies - with 30'000 people you fight back.


So the question is... WHY? Why the effort?

I can't think of any other logical reason, than a cosmic event. The earth was hit by something 3000 years ago, 6000 years ago, 12000 years ago, and so on and so forth.

All the signs are there, that it's coming back now. Should we start digging now? Like NOW?