The US government used to have huge bunkers in the middle of mountains and such for government continuity purposes in case of nuclear war, but as more powerful nuclear weapons were created it became clear that these bunkers would get destroyed, so they were quietly decommissioned.
Threads[1] is by far the most horrifying depiction I've seen of what nuclear war would actually be like, and as terrifying as it is it's probably overly optimistic. In a large-scale nuclear war the ones who perish instantly will be the lucky ones, and the "survivors" will just die a slower, more painful death.
[1] - https://www.youtube.com/watch?v=5Srqyd8B9gE
