Comment by Jedd

'Apart from terrorism ... or war, seismic activity, etc.'

I'm not sure where you're getting cheap from, or low-maintenance.

The above-ground stuff is locking future generations in for on-going maintenance for several centuries, perhaps longer. There's been think-tanks trying to work out how you just signpost such a place, given storage may exceed the expected lifetime of languages, and we'd want to be polite and at least contend with societal collapse.

It is hubris to observe that the many locations chosen now will remain 'distant from civilisation' for many centuries.