Subterranean fiction is a subgenre of adventure fiction, science fiction, or fantasy which focuses on fictional underground settings, sometimes at the center of the Earth or otherwise deep below the surface. The genre is based on, and has in turn influenced, the Hollow Earth theory. The earliest works in the genre were Enlightenment-era philosophical or allegorical works, in which the underground setting was often largely incidental. In the late 19th century, however, more pseudoscientific or proto-science-fictional motifs gained prevalence. Common themes have included a depiction of the underground world as more primitive than the surface, either culturally, technologically or biologically, or in some combination thereof. The former cases usually see the setting used as a venue for sword-and-sorcery fiction, while the latter often features cryptids or creatures extinct on the surface, such as dinosaurs or archaic humans. A less frequent theme has the underground world much more technologically advanced than the surface one, typically either as the refugium of a lost civilization, or (more rarely) as a secret base for space aliens. Literature In Dante Alighieri's Divine Comedy poem Inferno, Hell is a vast cavern and the narrator travels through the center of the Earth and out the other side to Purgatory. In Ludvig Holberg's 1741 novel Nicolai Klimii iter subterraneum (Niels Klim's Underground Travels), Nicolai Klim falls through a cave while spelunking and spends several years living on both a smaller globe within and the inside of the outer shell. Giacomo Casanova's 1788 Icosaméron is a 5-volume, 1,800-page story of a brother and sister who fall into the Earth and discover the subterranean utopia of the Mégamicres, a race of multicolored, hermaphroditic dwarfs. An early science-fiction work called Symzonia: A Voyage of Discovery by a "Captain Adam Seaborn" appeared in print in 1820. In the story, Captain Seaborn leads a group of travelers into the concave inner surface of the Earth. There, they discover an inner continent, which they name Symzonia after John Cleves Symmes, Jr. The story obviously reflected the ideas of Symmes, and some have claimed Symmes as the real author. Other researchers say it deliberately satirized Symmes's ideas, and think they have identified the author as an early American author named Nathaniel Ames (see Lang, Hans-Joachim and Benjamin Lease. "The Authorship of Symzonia: The Case for Nathanial Ames" New England Quarterly, June 1975, page 241–252). Faddei Bulgarin's short satirical tale "Improbable Tall-Tale, or Journey to the