Etidorhpa; or, The End of Earth: The Strange History of a Mysterious Being and the Account of a Remarkable Journey

This book purports to be a manuscript dictated by a strange being named I-Am-The-Man to a man named Llewyllyn Drury. Drury's adventure culminates in a trek through a cave in Kentucky into the core of the earth. It blends passages on the nature of physical phenomena, such as gravity and volcanoes, with spiritualist speculation and adventure-story elements (like traversing a landscape of giant mushrooms).