One of the most persistent crackpot ideas in astronomy is the notion that Earth’s Moon is hollow—an alien construct cleverly disguised as a cratered natural satellite. Apart from lurid tales of science fiction, this has actually been cited as the explanation for a variety of scientific findings about the Moon, including its initially apparently anomalous moment of inertia, the shape of large craters, which appear as if their depth was limited by encountering a hard shell below the surface, and the unusual seismic response to shallow Moonquakes, which were described as the Moon “ringing like a bell”.
For a deep dive (or is it wallow?) in Moonbat crackpottery, see George Leoard’s 1977 book Somebody Else Is on the Moon, which has been preserved at the Internet Archive.