Etidorhpa

Take a Victorian scifi premise, say, a trip to the center of the earth, and by the way, it's hollow. Add a tale of a soul condemned by the Illuminati to a perilous underground quest to find the Goddess of Love (spoiler alert: spell Aphrodite backwards). Top it off with a wild magic mushroom trip. That's Etidorhpa! This may be the very source of the 'adepts living in hollow earth who abduct humans' meme, later developed by Ray Palmer, and many others. The book is larded with long passages of speculative science. The structure of the hollow earth and the effects of gravitation at various places is much better worked out than some of the 'nonfiction' hollow earth books. The journey of 'I-am-the-man' is a not-so-subtle allegory of spiritual progression to being a disembodied adept. Along the way he loses his youth, loses sunlight, becomes weightless, stops breathing, can hear without ears, then his heart stops, and still he lives. Each step is symbolic of a progression to a more ethereal plane of existence.