Hollow World

"Old man: Years ago, I climbed the mountains, even though it was forbidden.

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."

- Star Trek the Original Series, "For the World Is Hollow and I Have Touched the Sky"

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, Ultraterrestrials)...down there, possibly powered by a suitably sized sun in the center.

The most known early example is Jules Verne's A 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).

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. Compare Beneath the Earth, Dyson Sphere. When the inhabitants don't know they're in a hollow world, it may become City in A Bottle.

Anime & Manga

 * The Fushigiboshi in Fushigi Boshi no Futago Hime, which gives it its name (which translates to "Mysterious 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.

Comicbooks

 * In The DCU, Skartaris was originally inside a Hollow Earth. A later Retcon changed it into an alternate dimension enetrable 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 BPRD story called "Hollow Earth". It features mutant cavemen and what looks like Nazi submarines.
 * 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.

Films

 * The movie The Mole People featured a team of scientists going down, down beneath the surface of the Earth to find another society of humans, along with the eponymous Mole People.

Literature

 * Journey to The Centre 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 mainstream one of several speculated scientific explanations 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 at Earth's Core.
 * 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 Heavy Planet was thought by its inhabitants to be bowl-shaped. They were incorrect (it was actually a very flattened spheroid).
 * Hell in Philip José Farmer's Inside Outside. According to some characters, it used to be flat but changed as scientific knowledge advanced.
 * In the semi-sequel to The Time Machine, called The Time Ships (by Stephen Baxter), the time traveller 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 (part of Halo's Expanded Universe) has a portal to one of these.
 * 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 a hollow world (in a kind of matryoshka style).
 * Margarat Weis and Tracy Hickman's The Death Gate Cycle's second book, Elven Star, features one of these, where all the stars in the sky are.
 * The world of elemental Water is another example, with the slight difference that the islands are floating in air-bubbles in a vast sea - meaning that people travel around in submarines instead of airships - and the world of elemental Fire is another variation on this theme.
 * 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.
 * The Shellworld(s) in Iain M. Banks' 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. While the "hollow" area is small, being only the size of Earth, located at the core of the ship, the rest of the ship is full of tens of thousands of fuel tanks that can fit moons, and even more caverns designed for habitation.

Live-Action TV

 * 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.
 * The planet that it orbits, Epsilon 3, confines an enourmous machine inside it, lots of hollow space and some inhabitants.
 * Lexx's 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.

Tabletop Games

 * 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 D&D 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 "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 it's 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.
 * It's also been recently taken over by Nazis- to be precise, Manes formed at the end of WW 2 when the real Nazis were crushed and almost all faith in their ideology crumbled.
 * There is some mentions in the Warhammer 40000 rulebook about hollow worlds.
 * The plane of Mirrodin (formerly ) 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.

Videogames

 * 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 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.
 * FunCom's upcoming MMO The Secret World has a PVP feature called the battle for Hollow-Earth in which the different factions battle over Agartha which is either this or Beneath the Earth
 * 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,
 * Final Fantasy IV. It has a unique configuration in that, instead of two habitable surfaces opossed 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.
 * The final stage of Dragon Quest IV. Also used in Dragon Quest III.
 * Septerra Core takes place, as the name suggests, on seven massive continents or "Shells" whose orbit and level are controlled by a central core with mythical wish granting powers.
 * While Word of God remains silent about it, most fans interpret World of Warcraft's Elemental Plane to work in this manner.
 * 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.

Webcomics

 * 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
 * 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.

Web Originals

 * 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 Dot 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,.
 * 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

Western Animation

 * The city of Arkadia and the other "strata" in Spartakus and The Sun Beneath The Sea.

Real Life

 * As alluded to above, this was an early theory of the Earth's structure dating back to the 1800. 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 believe 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.
 * Edmond 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 a 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.