If an ELE hits, few would be around to make even that short trip to the bunker.
Have you looked into or thought about "breakaway civilization"?
From my brief research, "breakaway civilization" is a keyword mostly associated with some extreme conspiracy theories concerning the Nazis and the inventions of Nikola Tesla?
I highly recommend watching various documentaries on them. I think I have consumed every sub dock I could find online.
https://www.youtube.com/watch?v=wHIS1I9tv78 is a great one. Also, note that the engineering needed to make the Global Explorer was new and significant. I think that with the making of that ship, the CIA gained a lot of know-how for sea-ops, which likely played at least a minor factor into their ability to make requirements/requests into the design of the Jimmy Carter sub, which is largely believed to be the sub they use to splice undersea cables/or cut them...
There's quite a lot of semiserious discussion of exactly this question on the internet; this is just a rough paraphrase of the consensus. It can make for a fun couple hours googling.
Humans live in a ridiculous variety of environments as it is - an extinction event would have to be utterly catastrophic to wipe out all breeding populations.
If it's possible, a few people will go just about anywhere. Islands, deserts, mountains, poles...
Mercury, Venus, Earth, then Mars.
(Mars is further from the sun and gets ~60% of our sunlight)
Question is why live somewhere that's tougher than the top of MT Everest or either pole? Guess a population big enough to repopulate earth in case of an extinction level event would be a reasonable safeguard.