The notion that the Earth might be hollow or at least cavernous inside and filled with numberless underground worlds (like volcanic and tectonic caves, sea caves, glacier caves, eolian caves, granite and other bedrock shelters, etc.) has fascinated humanity perhaps from times immemorial. The Hollow-Earth model, however, was formulated more clearly only in rather recent times (at best, it is only a few centuries old). John Cleves Symmes, in his science-fictional novel Symzonia: a voyage of discovery (1820), proposed such a model of Hollow Earth as inhabited inside by Symzonians (citizens of inner-Earth Symzonia) and Belzubians (degenerate Symzonians, living as exiles both in the inner-Earth Belzubia and on the outer Earth). The present paper presents a larger context in which Symmes's Symzonia came to life, by tracing a few romantic, alchemical, and philosophical sources (that may have been more or less its foundations) and echoes (that it may have more or less generated). Although generally underrated and neglected, Symmes's novel remains today relevant in science-fiction studies at least because it constitutes the first American SF novel, which saw the light of print only two years after the publication of Mary Shelley's novel Frankenstein; or, The modern Prometheus (1818), which is currently widely considered to be the first work of science fiction (see Aldiss & Wingrove 1988). What seems undoubtful is that the Hollow-Earth model of Symzonia deeply influenced E. A. Poe, who is now acknowledged as one of the initiators of the SF genre (see at least Beaver's 1976 edition of Poe's science fiction; and Tresch 2004). Also, it appears Symmes took his science-fiction notion of Hollow Earth to be a true reality, which is why Symzonia (1820) can be seen as a precursor for Willis George Emerson's equally mysterious (and perhaps equally neglected) narrative entitled The Smoky God, or A voyage to the Inner World (1908), which is hereby also briefly presented for comparative purposes as proto-science fiction. Interestingly, the first Superman movie (Superman and the Mole Man, 1951) is a Hollow-Earth narrative. In itself this constitutes yet another element that shows just how perennial and attractive this idea has continued to be. In this context, it becomes plausible to regard Symzonia (1820) as a groundbreaking American proto-SF narrative that initiated in literature a fictionary paradigm, later leading to Poe's mystery science fictions, Emerson's fantastic Hollow-Earth narrative, and the entire SF "school" of Hollow-Earth and underground stories (for an extended list of contributors to this significant SF genre, see Clute & Nicholls 1993). [ABSTRACT FROM AUTHOR]