It’s a setting in young adult literature that has crept into twenty-first century works almost unnoticed, but since the year 2000 there have been numerous stories set in underground realms. The Underland Chronicles, the Tunnels Series, the City of Ember quartet, and more stand-alone works use the terra cava chronotope as a space in which to set the adventures of young protagonists stumbling through dark caves as treacherous as puberty.
The Michael Printz award-winning novel The White Darkness (2005) by Geraldine McCaughrean is the only modern work that references John Symmes’s theory, starting with the narrator Symone “Sym” Wates whose very name invokes John Cleves Symmes. The titles itself and unfolding plot parallels Conrad’s Heart of Darkness and Poe’s Narrative of Arthur Gordon Pym. Accompanying her slightly mad Uncle Victor to the Antarctic in search of Symmes’s Hole in the Pole, Sym comes to realize that her uncle, like Kurtz, is quite mad. There is no polar opening to a subterranean world. Uncle Victor plans to leave Sym in his theorized hollow civilization, one he envisions as “more advanced” scientifically and politically (p 272), to be a permanent settler. Sym laments, “I thought Uncle Victor loved me. But he only wants to drop me in a hole in the ground – sacrifice me to the great god Symmes. It’s Symmes He loves. The whole idea of Symmes” (p 277). The white darkness takes Uncle Victor into a crevasse from which there is no return, but Sym resists and fights her way to rescue.
This is the one instance where no magic, no extraordinary science, actually exists in YA terra cava. Symmes’s theory and the history of Antarctic exploration are instead a framework for Sym’s journey, physically and spiritually. “Literature for children has to tread a careful path between a need to be sufficiently overreferential in its intertextual gap filling so as not to lose its readers, and the need to leave enough intertextual space and to be sufficiently stylistically challenging to allow readers free intertextual interplay” (p 135). Young adult readers are presented with nineteenth century work in a twenty-first century narrator to whom they might relate.
In the opposite vein, in the realm of magical realism, John “Captain Jack Harkness” Barrowman and his sister Carole Barrowman developed the fantastically skilled animist twins Matt and Emily Calder in the simply-titled Hollow Earth (2012). More fantasy than science fiction, it takes a more ethereal approach to the terra cava, like Etidorhpa over a century before. The Hollow Earth is “a supernatural space in the earth, a shadowy home for all the demons and monsters ever imagined” (p 57). Keeping these imagined nightmare creatures locked in the Hollow Earth is the mission of the eponymous secret society. It is not pure fantasy, as like Harry Potter the story is set in contemporary Britain; the narrative instead simply adds another layer to our known reality. “Magic realism, with its intrusion of the fantastic into real-world scenarios, presents an eloquent argument for the reinvigoration of animistic thinking in a world disenchanted by scientific explanation” (p 80).  Where animism is defined as the belief in souls occupying the universe beyond just human bodies, the twins are cast as Animare, “a supernaturally gifted artist who has such a powerful imagination that they can alter reality when they paint or draw. Simply put, if they choose to do so, an Animare can animate their own art” (pp 75-6). The Calder twins give soul to the world around them via imagination. The title is drawn from a factionalized Scottish artist called Duncan Fox, whose painting “Hollow Earth” is “said to depict the entrance to a mythical purgatory where all the beasts and demons ever imagined are trapper” (p 239). The nefarious plans of the Hollow Earth Society are to use the twins to open the gateway to this realm and unleash all the horrors of imagination onto the world. As must always happen, the heroes win, but at a cost, including the disappearance of their mother.
Another recent entry in the field is Emilie & The Hollow World (2013) by Martha Wells. Embracing the more recent fashion of steampunk and littered with a smattering of magic, Emilie’s world is not our own. Emilie serves as the reader’s avatar, learning at the same time we are, when she accidentally ends up on a ship bound for the hollow earth in search of the expedition leader’s missing father. (Paternal absence becomes a recurring theme.) Reviews of the novel tend to avoid any commentary on setting, focusing instead on other shortcomings., Though couched in the style and theme of Jules Verne, Verne would never have approved of such an unscientific novel. And by being set in a fantasy world entirely removed from our contemporary one, Symmes and his cohort are excluded from any mention in the narrative.
Suzanne Collins’s “Underland Chronicles” (2003-2007) is not as well-known as The Hunger Games, but many of the themes of responsibility, warfare, and the burdens of heroism remain. Gregor, the Overlander, and his toddler sister Boots fall through a vent in the laundry room in the basement of their New York City apartment and into the Underland. Like many previous terra cava narratives, this is a land inhabited by gigantic and ancient flora and fauna, and human refugees who fled the surface centuries earlier (like Beneath Your Very Boots). Generations deprived of natural sunlight are now devoid of melanin and speak archaic English. Gregor and his family are tied into the old prophecies of Bartholomew of Sandwich, the founder of Regalia. While the series is intended for younger readers than The Hunger Games, many critics have noted the same progression of protagonist, from innocent child to traumatised warrior who no longer feels at home in any world. Gregor, having survived the prophecy of his death, is deposited back on the Overland, unsure how to fit back in to normal life.
Perhaps the most successful juvenile terra cava series is “Tunnels” by Roderick Gordon and Brian Williams, stretching over six books. [It was my nephew, of all people, who alerted me to the series while briefing me about his after-school leisure reading. His mother was having difficulty getting copies of the books for him to read because they were constantly checked out of the school and local libraries, hinting at the popularity of the series.] The writing duo employ the semi-permeable Earth premise rather than one of Symmes Holes. But like the “Underland” books, it starts with the case of a missing father and Will Burrows (the name no doubt an allusion to the subject matter), with help from his friend Chester, stumbling upon an underground city, the Styx, and a conspiracy to destroy the humans who live on the surface.
The tunnels do eventually lead to the centre of the Earth in book 3, Freefall. The descriptors are similar to those in Nineteenth century terra cava: low gravity (p 24), abundant fungus (the entire place is fungi), an interior sun (p 548), and once-extinct creatures. To keep juvenile readers in the metatextual setting, Will’s father takes a short detour into the history of hollow earth theory, starting with Edmund Halley’s proposal and then Symmes’s revival of it, and even Dr Raymond Bernard: “Back in the sixties, some oddball professor claimed that a technologically advanced race inhabited the inner world, and that they had flying saucers” (p 491). This information is not necessary to the narrative, but as the world the authors have created has some basis in the real world, it serves almost as an educational tool, or at least a trivia question that might one day prove useful.
Children and young adults reading these narratives are not going to know the source of the terra cava setting (unless they are frequenters of this blog, or possess a masochistic interest in obscure Nineteenth century American novels) but the success and preponderance of these novels also indicates that they are not perturbed by the possibility of an interior world. But like the best young adult bildungsroman, the adventures had by the juvenile characters mature them in mind and body.
 Wilkie, Christine. “Relating Texts: Intertextuality” in Understanding Children’s Literature, ed. Peter Hunt. London: Routledge, 1999. Pp 130-137.
 Coates, Karen. “Fantasy” in The Routledge Companion to Children’s Literature, ed. David Rudd. London: Routledge, pp75-86.
 North, Phoebe. “Review: Emilie and the Hollow World by Marth Wells.” <http://strangehorizons.com/non-fiction/reviews/emilie-and-the-hollow-world-by-martha-wells/> Accessed 10/23/17.
 Roy, Leila. “Ride a Magical Submarine to ‘Emilie and the Hollow World’.” <https://www.kirkusreviews.com/features/ride-magical-submarine-emilie-and-hollow-world/> Accessed 10/23/17.