Do these populations currently exist? Or are you suggesting we build them?

Have you looked into or thought about "breakaway civilization"?

I doubt such populations currently exist, though secrecy would be a requirement to defend against active attacks, so I suppose anything is possible. I'm also not necessarily suggesting we do this, but for those interested in colonizing other planets solely as insurance against major life impacting events on Earth it seems like this would be a more feasible near-term goal to achieve. It would also be an excellent test of the systems we'd need to deploy in the even more remote and hostile environments of other planets.

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 would say some nuclear submarines qualify.

I love subs - they are awe-inspiring, amazing feats of engineering and terrifying weapons.

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...

https://en.wikipedia.org/wiki/Project_Azorian

The sad thing about the end of the cold war is that the governments are spending less resources on establishing such hidden underground societies