A fun idea but, like everything else these days, ruined racism and Nazis.
One of the great things about scouring sites like Project Gutenberg for free e-books of old speculative fiction, is that you can get a window onto out-there ideas written by respected authors. Perhaps the best example is Journey to the Centre of the Earth, by Jules Verne. This classic is an example of the “Hollow Earth” theory in action. Hollow Earth proponents expounded that there was a vast subterranean world beneath the Earth’s outer layer. Verne’s fiction envisioned a realm large enough to contain underground seas.
This wasn’t just a one-off story idea or short-lived crackpot fad. In the 19th century, it was a semi-serious theory and it resulted in some truly fun and imaginative fiction.