Simple enough question for all of us lore enthusiasts. You can stockpile your secrets, books, etc. anywhere safe from harm.
Where would it be? Why there? I'll keep my answers for a few days after this to see where the totals lay.
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Mount Everest. Freezing to the point of near inhospitable, remote, high vantage point. Make a vault out of one of the caves, seal it off from the outside weather, there you go. If the city gets nuked or something, at least the vault will be safe from harm.