A friend recently shared Atlas Obscura’s post on Hollow Earth theories. At once fun (because it is always entertaining to look at the crazy stuff folks once thunk, plus it’s a neat setting for fantastical tales) and terrifying (given that there aren’t only the flat-earthers and young-earthers out there [confession: I was one of the latter for a period as a child], but also the hollow-earthers). The article mentions Dante’s Inferno as a potential first-visualization of such an idea (I believe the genealogy of the idea has been traced back further, but hey, not my specialty), stops briefly at Halley, then skips on to 19th century pseudo-science, before getting into science fiction from Jules Verne onwards–which completely misses my favorite hollow earth story, Niels Klim’s Journey Under the Ground (links to various digital transcriptions of the English translation here). I’ve taught Niels Klim at least 4 times now, in various incarnations of my “Scandinavian Other Worlds” course (somewhat an overview of the history of Scandinavian literature, somewhat an exploration of different variations of the theme “Other Worlds”), though I am not an early-modernist (I’m a [Scandinavian] medievalist first, maybe a Scandi folklorist after that, then a general Scandinavianist), so the info provided below is brief and just a matter of a few things I found helpful/interesting/insightful when teaching it.

Image from Wikipedia
The story, from the same period as Gulliver’s Travels and clearly influenced by the same (though I’ve heard one person suggest it might go the other way, positing a very early Holbergian draft…) follows upwardly-mobile Niels Klim, who, on attempting to explore a mysterious cave in the mountains (and Scandinavian legend tradition re: underground populations of “under-earthers” is a relevant echo here, even if the story here opts for a very different direction), falls deep into the earth, emerging into the center of the earth, which consists of three parts–the lands on the firmament (the underside of the crust), a mini-sun in the middle, and a small planet (later we learn it is called Nazar) circling that sun. Niels is brought to the surface of the planet by a giant eagle, and there meets the inhabitants, who are sentient trees.
The larger narrative can be divided up geographically to a degree, and while Holberg’s point with the whole, apart from, well, entertainment, is fairly polemical, his strategies for his polemicizing shift from utopian to satirical with each location/section–though I suppose both elements are active here and there throughout: utopian in Holberg’s visions of social perfection, satirical in his biting commentary on contemporary Denmark and Europe.

Haven’t consulted the text in a while, so no promises this is an accurate depiction–but hey, close enough to an illustrious many-branched Potuan maiden.
First we have Niels’ initial stay in Potu, a country on the planet Nazar. This is an explicitly Utopian portion of the narrative, as may be clear from the name (Potu is derived from Utop[ia] in reverse–also, apart from this name, the language of the Potuans might be considered an early conlang, though I don’t know whether it was a serious enough construction to really be labeled such…). We get a first glimpse here of Niels’ role as buffoon, a role Holberg would use in his comedic plays as well–a particular characteristic would be taken to extremes in a buffoonish character, or such a character would seek to live outside his proper place and abilities, and so would be exposed to ridicule, such that the audience could point and laugh and say “Oh, OK, that is definitely not the right way to do things, is it…” (so not satire on specific real-world people or institutions, but on generally attitudes, behaviors, etc), but here this is primarily in terms of Niels as European foil to this logical and perfect society of intelligent trees. Examples of the perfection of the Potuans: no arguing allowed over religious points, and the religious outlook is vaguely Deist (so no Catholics and Protestants burning each other); apart from a hereditary ruler (this is the age of Absolute Monarchy and Villainous Aristocrats, after all) all jobs are assigned according to who can best do them (instead of matching the prestige of the job with the prestige of the person); as a development of the latter point, we get a pretty gender-progressive stance from Holberg (not Klim, alas) as the Potuans think it ridiculous to exclude women from, for example, prestigious government posts, so long as a given woman is most suited to that particular job; and introducing any change to society must be brought to the learned to consider, and if rejected the innovator will be executed–so that people will only offer an innovation if they are really very certain about it. Careful thought and consideration is the name of the game here, and Niels Klim is too hasty to even listen to the rules, and from the start the Potuans pity him, and he bridles under his reduced estate (ie, no longer so upwardly mobile–hm, maybe this subterranean position has a figurative connection to his career…). This brings us to the next section…
Unhappy with his humble position as courier (since, not being a tree, he can move quite fast), he convinces the king to let him go on a journey around the planet, surveying the other societies there. Here we might say we’ve gone Dystopian, though perhaps it is better to understand it as more satirical education via buffoonery, just projected on to the level of entire nations. Each place he visits has one particular thing taken to extremes (again, in true buffoon fashion, contradicting the Golden Mean)–a place where women are not just equal, but in fact in charge (which nicely illustrates how lame it is to actually be a woman in a patriarchal society); a place where everyone is a philosopher; a place where people live too long; a place where people know when they will die; etc. And of course, while many of these might be taken to derive from some vision of perfection (“wouldn’t it be nice if everyone…”), put into practice we see that nothing taken to extremes is good. So there!
This ends with him returning to Potu, but he is still unhappy–so he decides he will make a name for himself by introducing an innovation! Sure, it means risking his life, but a clever, upwardly mobile Norwegian boy like himself (OK, my blog title says Danish, but Norway was under Denmark at the time, so whatever) should have no trouble, right? So he suggests that women not be allowed to hold office. Well, it doesn’t go well, but because he is a stupid foreigner (not in so many words, but that is essentially it) they decide to exile him to the firmament instead–the inside surface of the earth’s crust. This is done using eagles of the sort that initially brought him.

Image from Wikipedia
On the surface we can still detect some dystopian elements and buffoon-at-the-level-of-nations polemicizing, but to a large degree this is where we finally actually start getting a narrative interesting in its own right, moving into adventure mode, and later into conquering hero mode–but all still very much a parody, and involving countries of apes, of tigers, of horses, and other animals, and of very primitive humans, along with our hero going from clever courtier to galley slave to “Yankee In King Arthur’s Court”-style antics that make him a celebrated, and then cruel, and, frankly, stupid conqueror and emperor. And OK, we do get one more intriguing bit of explicit satirical commentary as Klim discovers a manuscript from an subterranean explorer who went up to explore the surface–there are of course many details relevant to that period in European history (it is targeted satire, of course), but it essentially comes down to “Holy crap, those Europeans–smh.”
Klim screws up and has to run (spoiler alert), and ends up getting blown by a wind back up through the hole he fell into–on exiting he is mistaken for the wandering Jew (another bit of folklore to add to the “underearthers” reference), but ends up finally being taken care of by an old acquaintance, who helps him get a minor position–but of course he will forever remember that he was once a magnificent ruler and has a queen and princely children somewhere in the earth below, and the story itself is framed by a preface purporting to answer the ridicule of those who say it is all, pardon my language, balderdash (not a Danish/Norwegian word, btw). I tend to assume this was all tongue-in-cheek and that no one was going to be mistaking this for anything but a polemical flight of fancy, but hey, not a period I usually work with.
Like the Atlas Obscura article points out, the Hollow Earth idea has been around a long time. Wikipedia seems to have a good summary (and actually mentions Holberg), but if I remember correctly, this edition/translation has a good introduction covering not only Holberg, and not only the story’s place in the Hollow Earth tradition, but in literary history more generally. Alas, it is not in print any more, and the used paper back versions are a bit pricey. 😦 But again, feel free to read it for free online!
[EDIT: Holy crap, to think that only days after I published this that whole election thing happened. I’ve got to say, our president elect seems to have stepped right out of a Holbergian satire. Klim-as-buffoon ranges from relatively harmless ridiculousness as he gets himself exiled for his attempt to capitalize on his ostensibly ingenious misogynistic policy recommendation to very harmful (=world war level) ridiculousness as this very small-minded and entitled man pushes the martial, imperial, and colonial programs of Europe to extremes in the subterranean world after managing to displace an emperor–and when he himself is displaced from his ill-gotten throne he is full of his tragic downfall, oblivious to his role as hyperbolic object lesson. I’m tempted to get into political cartooning… but I suppose a centuries-old Danish utopian satire is not going to be the most accessible allusion for US politics…]