I read something, years ago, that polar explorer Richard Byrd reported flying over Antarctica and suddenly seeing a green, tropical area with people hunting mastodon. I wonder if this account was conflated with the idea of a hollow earth? I searched the Net about this, but couldn’t find much outside of raving Hollow-Earth type websites. 
Tekeli-li comes from Poe, not Lovecraft. The Narrative of Arthur Gordon Pym
That’s true. (I read Poe’s story as well, years ago.) But from here:
And from the link robardin posted earlier:
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Wasn’t there some crap about people living in the sewers of New York or something similar? Guess it was more a society than a civilisation.
No, I read an account of that trip, and there was nothing like that.
In any case, there’s doubt that he really flew over the North Pole in 1926.
This anecdote is indeed recounted in the previously cited “Raymond Bernard’s’” “Hollow Earth.” BTW, I put “Raymond Bernard” in quotes, as that name is made up. Martin Gardner, in one of his books, actually IDed who the writer was. Unfortunately, I’ve since forgotten. Seems like the guy moved to Mexico or South America or something.