> I looked back into the science-fiction literature to try to find the earliest example of a story featuring a nonhuman industrial civilization on Earth. The earliest I could find was in a Doctor Who episode.“
> That 1970 episode of the classic TV series involves the present-day discovery of “Silurians” [...]
That can't possibly be the first example of that concept in sci-fi, can it? Nothing against Doctor Who, just seems unlikely. Trying to think through the early sci-fi I've heard of or read though and I'm not coming up with anything with that concept.