Today there are people who believe that the Earth is flat -- two hundred years ago, there were those who believed the Earth to be hollow, and inhabitable, or actually inhabited, on the inside. While Terry Pratchett’s Discworld, elephants and turtle and all, is fun, “Symzonia” in 1820 was (very likely) seriously meant to promote the project of an expedition to the Earth’s interior. It also uses the tale of such an expedition to deliver social criticism, by contrasting our world with the vision of a peaceful Utopian society of the “Internals.”
Admittedly, “Symzonia” is not particularly exciting as an adventure story, and its social criticism is muted, but for the history of fantasy literature this is an important book -- it took a century for the hollow Earth to be visited again, by Edgar Rice Burroughs in his Pellucidar series.
dunyazad-library.net/authors/captain-adam-seaborn.htm