Old man: Years ago, I climbed the mountains, even though it was forbidden.So, mainstream scientists today believe that the Earth under our feet has a lot of molten rock and metal filling it and have gathered a lot of pretty solid evidence for it. The only complication is that we've never been able to send a human down more than several miles to actually study it up close, largely because No One Could Survive That!. Which is why since times that are Older Than Radio, early scientists, writers and more than a few crackpots have believed that there just might be something...or indeed, someone (say, Ultra Terrestrials)...down there, possibly powered by a suitably sized sun in the center. The most known early example is Jules Verne's Journey to the Center of the Earth, although he likely drew from theories of his time. When science started to switch over to the modern view of Earth's composition the idea of the hollow earth became a Discredited Trope, but later generations of Speculative Fiction writers took up the concept and revitalized it. Sci-Fi works bring us hollow world concepts such as the Dyson Sphere, which is a Hollow World taken to a solar system scale, and other variations of artificially constructed worlds. Note that its usual configuration, with people walking about on the inner surface, wouldn't work; a hollow sphere has no net gravitational pull on any object inside it (although some theorists, such as John Symmes, claim that this actually could work due to the centrifugal force caused by the planet's rotation. However, it would still have to be very low, otherwise the planet itself would break apart note ). A related belief is that of "Concave Hollow Earth": that Earth is actually a hollow bubble inside an infinite mass of rock. A sub-trope of World Shapes and, in more modern works, an example of All Theories Are True. Compare Beneath the Earth, Dyson Sphere. When the inhabitants don't know they're in a hollow world, it may become a City in a Bottle or a Lost World.
Kirk: Why is it forbidden?
Old man: I am not sure, [writhes and gasps in pain from a control device] but things are not as they teach us. For the world is hollow and I have touched the sky.
Kirk: Why is it forbidden?
Old man: I am not sure, [writhes and gasps in pain from a control device] but things are not as they teach us. For the world is hollow and I have touched the sky.
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Anime & Manga
- The Fushigiboshi in Fushigiboshi No Futago Hime, which gives it its name (which translates to "Wonder Planet").
- Dr. Suzuki in Transformers Cybertron believes the world is hollow, with a hole at the North Pole leading to the inside. She's wrong, but there is a big massive cavern full of ancient Decepticons.
- This is a main plot point in Spider Riders. Hunter finds the Inner-World in the very first episode.
- One Doraemon comic centers around Nobita and Doraemon's discovery of a barren, hollow realm in the center of the Earth thanks to a gadget which warps any myth into reality in the eyes of its users. As you would expect from an adventurous kid and his multipurpose robot, the duo proceed to populate the space with life, including little clay people that rapidly form a fairly advanced civilization and eventually venerate the duo like gods.
- This is used in Maoyuu Maou Yuusha. The "no gravitational pull" aspect is got around by having the sun be a repulsing force instead, pushing things away from it toward the ground.
- The setting for most of Children Who Chase Lost Voices is Agartha, a massive underground world with a completely open sky. Any physical or geological impossibilities with this can seemingly be explained by the fact that God lives here.
- Endride is set in Endora, a fantasy kingdom inside a hollow Earth. There is a large green crystal floating in the center that serves as their sun. And somehow there is an open green sky there.
- In The DCU, Skartaris was originally inside a Hollow Earth. A later Retcon changed it into an alternate dimension that was accessed through gates at the Earth's poles.
- Another DCU example: Steve Conrad, a Golden Age adventurer, explored a Lost World called Mikishawm inside the Earth.
- Valerian and Laureline [a French comic]: The inhabitants of the "starless country" live inside a hollow planet. The core is their sun. And yes, somewhere a physicist is crying.
- There's a B.P.R.D. story called "Hollow Earth". It features mutant cavemen and what looks like Nazi submarines. However, it's not quite a traditional version - it's more like there is a network of subterranean caves in various places around the world where said mutant cavemen (really former servitors of the ancient Hyperboreans) live, and most of the world seems to be largely as we know it aside from that. Agartha and similar legends from Theosophy and the Thule Society occasionally crop up in the broader Hellboy universe but the truth is generally rather off from what the Theosophists thought.
- The spinoff comic Witchfinder has a malevolent spirit explicitly exploiting a group's beliefs in the Hollow Earth to feed off their collective life forces (and then their blood).
- In one issue of the Gold Key Underdog comic book of the 1970s, Underdog met many strange creatures, including a rock 'n' roll band, as he followed Simon Barsinister's drilling machine through the earth.
- In the Marvel Universe, Saturn's moon Titan is this: barren on the outside, fully inhabited on the inside.
- In the miniseries Batman Odyssey, Batman travels Beneath the Earth and finds it hollow and filled with dinosaurs, trolls, wizards, monsters, and so forth. Neal Adams is on board with this idea.
- In Society of Super-Heroes: Conquerors of the Counter-World #1, Immortal Man mentions in passing that he just walked back from inside the Earth.
- Atomic Robo makes it very clear that the Earth is not hollow, because that would be scientifically impossible. However, there is an extensive, undetectable planet-wide cave network populated by an alien silicon-based ecosystem, created when debris from Theia was captured by Earth's gravity and sank into the crust billions of years ago. note
Film - Animated
- In the Ice Age film series there is an underworld which dinosaurs still roam.
Film - Live Action
- Journey to the Center of the Earth implies that there is a second sun at the core of our own planet, meaning we live on the outside of such a sphere. At the time, before the discovery of radioactive elements, this was
the mainstreamone of several speculated scientificexplanations for the Earth's internal heat.
- Edgar Rice Burroughs cemented the concept in pulp with Pellucidar, an internal world where he set several of his stories, including a notable crossover with Tarzan in Tarzan at Earth's Core. He also wrote The Moon Maid, in which the first spacecraft from Earth to land on the Moon discovers that it is hollow, with a living internal world that can be reached by descending through certain craters.
- Choose Your Own Adventure: The Underground Kingdom, which had a star in the middle of the earth and an advanced civilization living inside of the crust.
- Likewise Through the Black Hole, in which the planets in the black hole appear to be spherical and smooth (and repel all other objects), but can be burrowed into with some difficultly to locate the lush wildlife and Sufficiently Advanced Aliens on the inside.
- Olaf Stapledon's Star Maker.
- Robert Silverberg's Across a Billion Years.
- Colin Kapp's Cageworld series.
- Bob Shaw's Orbitsville.
- Farthest Star
- Wall Around a Star— the inhabitants mainly lived on the outside of the Sphere (or in the various layers of the Dyson Shell).
- The eponymous structures in George Zebrowski's Macrolife.
- The John Gribbin novel Innervisions. This is meant to be a shock ending to the book, except the cover announces "The world was a sphere ... and they were inside it!"
- In the sci-fi novel Prisoners of Power by A. and B. Strugatsky, the inhabitants of the planet Saraksh are convinced that they live on the inner surface of a spherical cavity, due to the unusual optical properties of its atmosphere (the horizon looks like it is above the observer).
- Due to a combination of its gravitic and atmospheric oddities, the world of Mesklin in Hal Clement's hard sci fi classic Mission of Gravity was thought by its inhabitants to be bowl-shaped. They were incorrect (it was actually a very flattened spheroid).
- Hell in Philip Josť Farmer's Inside Outside. According to some characters, it used to be flat but changed as scientific knowledge advanced. It's later revealed, however, that this is false and that hell is a space station.
- In the semi-sequel to The Time Machine, called The Time Ships (by Stephen Baxter), the time traveler returns to the future once again, but finds it changed. The Morlocks are now "good" in this future, and are also incredibly advanced, having engineered a hollow sphere around the Sun slightly inside Earth's orbit. Morlocks live on the outside of this hollow sphere in the dark, while the Eloi live on the sunlit inside of it.
- Also by Stephen Baxter, a short story called "Shell" is set on a planet that has been folded in on itself in the fourth dimension. There is no sky — people looking up see the other side of the planet curving over them, as if it's a shell. When one character uses a hot-air balloon to explore the other side, she witnesses the "shell" flatten out and then become curved normally, while the land she just left curves into a shell over the sky.
- In the Artemis Fowl series, the Fairies moved to inside the Hollow Earth in recent times - that or Beneath the Earth depending on how you interpret it.
- Nehwon in Fritz Leiber's Fafhrd and the Gray Mouser stories is a concave hollow world.
- The interior of Onyx in Halo: Ghosts of Onyx has a portal to one of these. Specifically, it's a Forerunner Shield World.
- In one of Isaac Asimov's stories in the rather epic Robots/Empire/Foundation chronology, some of the inhabitants of Trantor believe the universe to be an infinite mass of earth and rock, punctuated by occasional life-bearing bubbles. It is worth noting that the truth is more along the lines of Beneath the Earth, but only the (increasingly rare and alienated) scientific and technical elite still believe this, with the others believing such a belief to be a quite frankly bizarre conspiracy, the details of which I can't quite remember.
- In Robert Rankin's The Greatest Show Off Earth, our world is the inner layer of a hollow world (in a kind of matryoshka style). The inhabitants of the outer layer are planning to plug up the holes at the poles because they are fed up with our pollution spilling out.
- The Death Gate Cycle, by Margaret Weis and Tracy Hickman.
- In second book, Elven Star, features the world of elemental Fire as one of these, where all the stars in the sky are the equivalent of gigantic lighthouse beacons.
- The world of elemental Water is another example, with the slight difference that the islands are floating in air-bubbles in a vast sea filling its volume, meaning that people travel around in submarines instead of airships.
- Even the world of elemental Air apparently is this according to the map, even though the "walls" presumably consist of some kind of force fields rather than solid matter. The final book implies that all the worlds exist in what used to be our solar system, and can be reached through mechanical rockets even after the Death Gate is closed — after you get through the planets' artificial crust, in any case.
- More Information Than You Require features a long section on the mole-manic societies dwelling within the hollow Earth, including a list of 700 mole-man names and countless allusions to many of the other works listed on this page.
- In the Tunnels series, the Earth's core is hollow and contains an Eden-like paradise called "The Garden of the Second Sun".
- The Indiana Jones novel Indiana Jones and the Hollow Earth is about this. More specifically, Indy discovers Ultima Thule (see "Real Life" examples below).
- Hollow Earth: The long and curious history of imagining strange lands, fantastical creatures, advanced civilizations, and marvelous machines below the Earth's surface by David Standish goes into detail on the fictions, theories, and wacky religions inspired by this trope, but even he doesn't bother listing all the stories based on this trope (it was, for example, quite popular in the 19th century to base utopian fiction inside a hollow earth).
- In John C. Wright's The Golden Transcedence, the agent of the Silent Oecumene claims that they live inside a black hole, which has been hollowed out and so exerts no gravitational pull on them.
- The White Darkness. A girl is dragged along by her eccentric uncle to find the entrance to the hollow earth in Antarctica. He's wrong, and quite insane.
- The Shellworld(s) in Iain M. Banks's Matter is essentially a nesting-doll series of these, although all the inhabitants live on the "outside" of each shell. What's on the inside? Why, artificial stars, some of which roll across the sky and some of which are fixed.
- Thomas Pynchon's Against The Day seems to have this. It's unclear.
- The Great Ship is a hollow starship, about the same size as Jupiter. It has a hollow core large enough to contain an entire world, Marrow. The rest of the ship is full of thousands of fuel tanks that can fit moons, and even more caverns designed for habitation.
- Umberto Eco's novel Foucault's Pendulum pokes fun at this trope, together with many others commonly believed by conspiracy theorists.
- The BookWorld in the Thursday Next series eventually reboots and turns into one of these in One Of Our Thursdays Is Missing. Used to be the various books were essentially pocket worlds floating in space independently of each other, only connected by a library acting as a dimensional nexus of sorts, but they now occupy the surface of a hollow world, with the various subdivisions of literature — Fiction, Poetry, Mythology, Non-Ficition, etcetera — each occupying a single continent.
- A variant in Nikolay Nosov's children's book Dunno on the Moon, where the Moon turns out to not only be hollow but actually housing a mini-Earth inside it. The only difference for the shorties (all people in the book series are shorties, small humans) living on the mini-Earth is that, at night, everything is dark. Some technobabble explains how they get any light at all during daytime (something about cosmic rays turning into visible light when passing through the Moon's crust). The Moon-shorties have no idea that they are living inside a planetoid, as they don't have rockets yet and, frankly, have no need to try to figure out how to get into space (being a thinly-veiled stand-in for Western capitalists, most Moon-shorties are greedy and corrupt and, as such, are uninterested in the advancement of their people; this is the first book where the concept of money is brought up, when an Earth-shortie goes to a restaurant and tries to leave without paying). Another big difference (that is actually a big plot-point) is that, while on regular Earth all fruits and vegetables are huge compared to the shorties (as it "bigger than the shorties" in some cases), all fruits and vegetable on the mini-Earth are in proportion to them. Thus, for those living on the mini-Earth, hunger is a real possibility. After explaining who he is to two Moon-shorties, Dunno tells them that their rocket on the Moon's surface is full of seeds of giant fruits and vegetables. The three of them try to market that idea and create a fund for building a rocket to get from mini-Earth to the Moon's surface and retrieve the seeds. Unfortunately, without "moonite" (an Expy of H. G. Wells's cavorite) that can only be found on the Moon's surface, building a rocket is extremely difficult, as shown by the second expedition sent from Earth (the rocket is much smaller and most of it is devoted to fuel; the first rocket, equipped with "moonite" only needed a little bit of thrust and was luxurious by comparison).
- The iconic example from Perry Rhodan would be Horror, a hollow world in six distinct layers that at least from the protagonists' perspective seemed to serve primarily as a planet-sized trap for unauthorized users of the recently-discovered intergalactic matter transmitter network connecting at least the Milky Way and the Andromeda Galaxy (plus some outliers) at the time. The "Mobys" from later in the same arc probably also qualify by virtue of being largely sleeping large-moon-sized anorganic lifeforms whose insides were frequently colonized by opportunistic aliens.
- In The World And Thorinn, there are many worlds within the Earth with the lower ones having less of a pull of gravity due to almost as much material being overhead as underfoot.
- They also turned up in Star Trek: The Next Generation and Star Trek: Voyager.
- TOS episode "For The World Is Hollow And I Have Touched The Sky" had a variation, a shell covering an artificial planetoid to hold the atmosphere in.
- The Babylon 5 station could technically be considered a "world" of this sort.
- Lexx's planet Fire has an inner surface with inexplicable bright daylight and Earth-normal gravity. Given its supernatural nature, one can only shrug.
- In Sanctuary it turns out that the Earth really is hollow. And the only people that knew were Jekyll and Hyde.
- It's actually populated by technologically-advanced humans and Abnormals who have migrated underground in order to escape the vampires ruling the world at the time.
- Doctor Who: The Fourth Doctor story "The Pirate Planet" gave this an interesting twist. Zanak was a gigantic hollow planet with enormous transmat engines at its core. It could literally materialize around smaller planets and squeeze them of their wealth and energies.
- The Clock Punk opening sequence to Game of Thrones takes place on at the very least a parabolic version of the world map of Westeros and Essos with an astrolabe floating above it functioning as both the sun and the Title Card for the show.
- The indie Hollow Earth Expedition is all about this, using the fluidity of the pulp genre to meld the hollow earth with Thule, Atlantis, and prehistoric times, and any sort of lost civilization, and the whole thing is discovered on the cusp of World War II. Hey, how else are you going to feed Nazis to dinosaurs?
- The Hollow World is a Dungeons & Dragons game setting inside another D&D game setting, being located inside the planet Mystara.
- Spelljammer got Herdspace described in The Maelstrom's Eye by Roger E. Moore — a crystal sphere with an internal surface (the size of a "small" solar system) covered with inhabitable landscape, much like a naturally-occurring Dyson Sphere.
- Pathfinder, as a nod to Pellucidar, has a inverted vault in its Underdark-analogue, complete with otherwise-extinct animals. It's more like a mini-world than an actual hollow planet, though; an ancient terrarium of Sufficiently Advanced...somethings.
- The World Builder's Guidebook, a supplement for 2nd Edition D&D, discussed variant World Shapes, including hollow worlds. Notably, it points out that the same blank globe-maps it provides for Earth-like spherical worlds are also perfectly usable for a hollow sphere.
- Genius: The Transgression has the Hollow Earth as one of its many bardos - pocket realities created when established science is proven wrong. From the presence of things like "brontosaurs" and Piltdown Men, it's implied to be a catch-all for every paleontological blunder ever committed.
- There is some mentions in the Warhammer 40,000 rulebook about hollow worlds.
- The plane of Mirrodin (formerly Argentum) in Magic: The Gathering is hollow, and mostly metallic. There are five channels, called lacuna, through which one of the planes five suns emerged from the mana core. There are five lacunae, one for each color of mana, and one for each of Mirrodin's five suns.
- The Synnibarr roleplaying game's titular planet is hollow. According to the backstory, Earth's sun was becoming unstable, and because Earth itself was physically unsuitable, the planet's stellar engineers took Mars and expanded it, hollowed it out, and terraformed it until it was completely unrecognizable to serve as a new home for the evacuated human race. Overlaps with That's No Moon!, as a suitably huge power generator was installed in Synnibarr's core to power the artificial planetary heating and atmosphere systems as well as the engine designed to propel the planet to a new star system to take up orbit in. After dozens of catastrophes, wars, and invasions during the 40,000 year journey, the inner core of the planet was abandoned, with the planetary machinery almost completely inaccessible at the top of a thousand-mile high mountain ascending to the planet's geographical core.
- Downplayed with the discontinued world of Tharun in The Dark Eye, which was actually an alternate universe (with its own gods and celestial bodies), whose only gates were reached through deep caverns. So it felt like a hollow earth, although it wasn't.
- The early BIONICLE stories take place in a universe that is entirely contained within the body of their creator god.
- Eternia, world of the Masters of the Universe, is a hollow planet with an entire "Subternia" existing below its surface, complete with a multi-armed giant in the center holding everything together.
- Terranigma has this at the beginning, and later the protagonist drops through a hole to find himself on the surface of Earth.
- The world of La Gias from Super Robot Wars (only visited in the Gaiden Game Lord Of Elemental, but referenced throughout the series) is like this. Its specifically in the center of the Earth, but its more of a magical dimension.
- Shin Megami Tensei III: Nocturne: At The End of the World as We Know It (roughly 30 minutes into the game), the city of Tokyo becomes the new world, with the central "sun" waxing and waning every few days.
- Atrea, the world in Aion was hollow with the Tower of Eternity through the center as the manifestation of the god Aion who provided the inside of the world with heat and light. However, after the Cataclysm, the center of the tower and a good deal of the planet's equator were destroyed leaving an apple-core shaped world held together only by magic. Although the planet orbits a nearby star, only one half of the planet gets any light from the star.
- Macbeth from the original Star Fox is stated to be a hollow planet, but in truth there is a core, albeit smaller than the crust that surrounds it. This makes a huge series of caverns that covers the entire planet. Andross had plans to turn Macbeth into a massive base, in other words, a naturally formed Death Star (without the superlaser).The actual stage even takes place with the Arwings flying around inside the planet.
- The Secret World features a transit network between the various locations across the world, built into the depths of the hollow earth; known as Agartha, said transportation is based around the roots of the World Tree itself, and is attended to by a number of hulking Golems and one eccentric Victorian stationmaster.
- The world of Dwarf Fortress may or may not be spherical, but it's hollow alright, as your dwarves may discover the hard way. Specifically, it has a sort of "Swiss cheese" layering of an unmineable rock called slade which is covered in semi-molten rock and adamantine, depending on where.
- Final Fantasy IV. It has a unique configuration in that, instead of two habitable surfaces opposed to each other, the Underworld IS the basic, solid planetary sphere, while the Overworld encases it. As proof, the Tower of Bab-Il rises from the Underworld and continues upwards through the Overworld. The Underworld is also considerably smaller, and, being located between the solid sphere and its shell, it lacks a Sun. The magma flowing through it is more than bright enough to do the job, though.
- Most of Final Fantasy XIII takes place on Cocoon, a Hollow World roughly the size of North America were it rolled up into a ball. Humanity resides on the inside surface area off the sphere, which is comprised of several distinct landmasses and oceans, and a giant freaking hole to nowhere. Cocoon floats in the lower atmosphere of Gran Pulse, a planet somewhat larger than Earth, given the apparent scale of Cocoon in comparison to it. Nothing about Cocoon makes sense scientifically, but it works anyway thanks to a million-some Physical Gods forcibly willing it to do things like float above Gran Pulse, and have a functional atmosphere, gravity, and a day/night cycle.
- The final area of Dragon Quest IV. Also used in Dragon Quest III where one of the big plot twists is The Reveal that the first two games took place inside an alternate Earth.
- Septerra Core takes place on seven massive continents or "Shells" whose orbit and level are controlled by a central core with mythical wish granting powers.
- Some of the planets in both Super Mario Galaxy and Super Mario Galaxy 2 resemble hollowed out spheres covered with huge holes and a black hole underneath. If you fall into the black hole, you die. Also, the boss battle against Kingfin takes place inside a hollow planet, and in the case of the final battle against Bowser, a hollow Sun.
- The Spin-Dig Galaxy level from Super Mario Galaxy 2 appears to be located inside one of these.
- Xardion has the Hollowsphere (Zikar), which is icy on the outside and jungle on the inside.
- In Halo, Forerunner shield worlds are this combined with Dyson Sphere. They were designed to shelter their inhabitants from the Flood.
- Schlock Mercenary has the Buuthandi, built by the F'sherl-Ganni. Due to math they're rather more like spherical solar sails counterbalanced with living habitats (drastically reducing the living space compared to a standard Dyson Sphere— only a few hundred thousand times Earth rather than millions), but allow them to tame an entire star as a source of energy and mass.
- In Homestuck The Battlefield has a very thick crust with a hollow center housing THE TUMOR (Before being removed) in its center.
- Reptilis Rex is based on the premise that Hollow Earth is real and that the Lizard Folk living in the center are forced to leave and integrate into human society with limited success.
- Axe Cop features a civilization of people living within the Moon rather than on top. The Moon Ninjas come from here.
- The Web site PsyPets has this as part of its Framing Device; you're a volunteer working for the Hollow Earth Research Group (HERG). However, "Hollow Earth" in this setting is not actually inside Earth; rather, it's just a name given to an alternate dimension because the researchers were reminded of the old myth. References to the actual "Hollow Earth" pop up sometimes, though.
- In AH.com: The Series, a Hollow Earth appears that combines Shout Outs and homages to just about every Hollow Earth in fiction. It first appears in "Dinos and Nazis and Deroes, Oh My!" and, in a Tear Jerker, is destroyed in "Harbingers".
- What Curiosity in the Structure: The Hollow Earth in Science by Duane Griffin, is a short historical paper surveying scientific thinking about the hollow earth up to the present day.
- The Hollow Earth Insider
- In The Salvation War, Heaven and Hell are both Hollow Worlds. This is partly due to the linguistic convention of saying that one is "on" earth but "in" hell or heaven. They have Alien Geometries in other ways as well; traveling in what appears to be a straight line on them results in a curved path.
- As alluded to above, this was an early theory of the Earth's structure dating back to the 1800s. And like the flat earth theory, some people on the fringes continued to advocate it.
- The Theosophical Society were believers - their philosophy held all religion to have some virtue, all races were equal and they would promote studies in science, philosophy and religion. Popular through the early 1900s.
- The Thule Society were less charitable - their beliefs influenced the Nazi doctrine of Aryan supremacy and Nazi mysticism.
- As recently as 2007, some guy got some attention for his fundraising for an expedition to the poles to enter the holes to the hollow earth supposedly located there.
- An unfalsifiable (and therefore scientifically irrelevant) claim is that the earth is in fact a hollow sphere with the universe inside and geometry transformed to match (i.e. the closer in you go, the smaller you get).
- David Icke believes this is where the Nazi leadership escaped to. He also believes that the holes are enormous and that's why commercial planes never fly near the poles. (Planes frequently fly over the poles as a way to cut down on travel times. It's easier to fly to Russia from Canada by flying over the poles than going west.)
- Edmund Halley (of Halley's Comet fame) was one of the earliest proponents of the Hollow Earth as a scientific theory rather than a legend or religious belief. He speculated that Earth was made of several hollow spheres (akin to Russian Nesting Dolls) that rotated independently of each other. He came up with this theory as a way of accounting for variations in the behavior of magnetic compasses; his idea was that something below the surface (the aforementioned spheres) was causing interference. (To be completely fair, it's not that far off from the modern theory of a dynamo generated by a rotating liquid core.)
- In the 1820s, Hollow Earth proponent John Symmes lobbied the United States government to fund an expedition, arguing that the U.S. could conduct valuable trade with the people living inside the Earth. President John Quincy Adams was convinced, but the project was cancelled by his successor, Andrew Jackson. Due to to Jackson's lack of formal education and hillbilly image, it's been joked that he canceled the expedition due to believing the Earth was flat.
- The closest (known) Real Life things to a hollow world are Saturn's moon Hyperion, an irregularly shaped body that is said to have a 40% of its volume as empty space, and similar rubble piles as Mars' largest moon Phobos.