I actually recall a real, not fake, scientific paper (maybe it was an editorial or something but it was legit) that discussed the possibility of a hollow earth.

This was like 15 years or so ago so my memory is a bit hazy.

The thesis was that the working hypothesis of how magnetism and other evidence around the earth with layers can also point to a hollow earth. There was a lot of physics and math. The paper was serious but not like “I have cited sources” but more of an idea.

I suspect that, like most conspiracy theories, the hollow moon took that idea and perverted it beyond recognition.