Beneath the Earth



"All the roots hang down, swing from town to town They are marching around down under your boots ''All the trucks unload beyond the gopher holes There's a world going on underground."

- Tom Waits, "Underground"

A very short distance beneath our feet, there dwell fantastic beings, societies and terrors.

Those who live Beneath the Earth are often exiles from the World Above. They fled either to create a new home for themselves, or to harbor their grudge for revenge (depending on how well they did). Alternately, they may have fled to escape The End Of Their World As They Knew It. If they had better technology or more resources, they might have built an Elaborate Underground Base, but they don't, so they have to make do with simple caves and tunnels.

The Urban area version of this trope is a remarkably livable sewer system. Sewers are surprisingly clean and warm, relatively speaking, with good lighting and electricity access. Maintenance crews never stumble across the living quarters, nor do power companies realize the drain. (Sewer Dwellers don't pay electricity bills.) New York has an especially crowded sewer system.

Go a few kilometers deeper, and the Earth's crust is filled with spacious caverns. The really lucky Beneath the Earth dwellers will have a Lost World thing going, with tropical flora and fauna in abundance (although occasionally with monsters like dinosaurs).

Not so lucky ones get gloom, fungus and lava. (They're the ones who usually want revenge.) Often based on the "Morlocks" in H. G. Wells' book The Time Machine. Particularly well-to-do ones will build an Elaborate Underground Base instead.

Often found side-by-side in with the Underground Level and Absurdly Spacious Sewer. In mythology, folklore, and fantasy, this is typically where you'll find The Underworld. (Or that other place.) See also Mouse World, which is basically this but on a smaller scale, and Dug Too Deep. The extreme version is the whole Hollow World.

Anime and Manga

 * Getter Robo: The Dinosaur Empire were evolved dinosaurs were forced to hide underground since Earth was being bathed by a type of cosmic radiation -the Getter rays- was killing them.
 * Keroro Gunsou gives us "Side 6", and underground city which functions as home and refuge for all alien immigrants on Earth. Apart from being underground, it looks exactly like any other street in Tokyo, with Expies of known locations like Akihabara.
 * Kotetsu Jeeg: The Yamatai Kingdom were a subterranean civilization, too. They had attempted conquering Japan in ancients times but they were defeated and fled underground. Big Bad Queen Himika definitely harbored a grudge and longed for revenge.
 * Mazinger Z and Great Mazinger: Not surprisingly, Mazinger Z was the one introduced this trope in Humongous Mecha anime. The Mykene were a civilization lived and thrived in the Greek island of Bardos millennia ago, using mechanical giants blast fire from their chests to protecting their land from invaders. An earthquake destroyed their island and forced them to seek shelter underground. They lived below Earth for millennia, building their cities in networks of subterranean tunnels and caverns and grafting their bodies into Humongous Mecha to survive. Unlike fromn other examples from this trope they did not strictly harbored a grudge against humans... but they did not like them, nonetheless.
 * In Tengen Toppa Gurren Lagann, all of humanity has been forced to live underground by the Spiral King Lordgenome. The inhabitants of Jiha village, at least, have been down there so long they no longer believe the surface exists.

Comic Books

 * There was a group called "The Morlocks" in X-Men who lived in the sewers. They were mutants who couldn't pass for normal humans.
 * Originally the Morlocks in "X-Men" chose to have their physical appearance altered by Masque as a sign that they had rejected the surface world.
 * Much deeper than the Morlocks lies Subterrannea, inhabited by yellow-skinned, weak-willed humanoids and ruled by various humans who've gone there over the centuries—most notably the Mole Man, a Fantastic Four villain.
 * Also from Marvel is the Lava Men from The Avengers.

Film

 * The sewer-dwelling hoboes in the movie "C.H.U.D" mutated into the titular Cannibalistic Humanoid Underground Dwellers.
 * In Demolition Man, the anarchist/rebel types led by Dennis Leary's character live in tunnels beneath the city of San Angeles.
 * All the way back to Metropolis. The rich live in a bright shiny modern city and the workers live in dreary underground apartments accessed by huge elevators crammed full of people at the end of each shift.
 * From the Star Wars Expanded Universe, though not seen in the movies, there are Coruscant and Taris, and any other city-planets or very large cities that get mentioned. The rich live at the tops of skyscrapers, the poor live much lower down in those skyscrapers, in slums crushed under the weight of and with the sun blocked out by said skyscrapers. And then there are the outcasts, who live either in caves, sub-basements, or the Absurdly Spacious Sewer systems required by such large populations. Star Wars is much more of a crapsack 'verse than the movies show.
 * Not to mention the lower levels of the giant forests of Kashyyyk, referred to as a 'layered deathtrap' with seven levels. The Wookies only live at the top level, with level four or lower very seldomly ventured to.
 * The Matrix. Zion, Humanity's last civilization free of the Matrix is located near the Earth's core for its warmth.
 * The Lord of the Rings. Moria, the subterranean dwelling of the Longbeards Dwarf clan under the Misty Mountains.

Literature

 * The Dresden Files has Undertown, an area under Chicago full of mildly-radioactive supernatural beasties.
 * Neverwhere by Neil Gaiman (based on the Live Action TV miniseries written by him, which see).
 * The plot of the book The Relic centers around a monster living in the tunnels beneath a museum, and its sequel Reliquary had mutants dwelling in the deepest parts of New York's sewers.
 * In Neal Shusterman's book Downsiders there is a whole community of people who live under New York City in.
 * Suggested in the novel War of the Worlds, as the artilleryman briefly involves the hero in a plan to form a literally underground resistance movement against the Martians based in the London sewers (which, he says, would be swept clean by the rain once they've fallen into disuse), using tunnels connected to cellars throughout the city. Nothing, besides a single half-dug trench, actually comes of his plan.
 * Interesting example in Narnia, where there is a "Deep Realm" beneath the surface of the (flat) earth ruled by the Lady of the Green Kirtle, but it turns out that many of the enchanted slaves there are actually from an even deeper realm called Bism, where gemstones are alive. Once roused from their enchantment, they find the so-called Deep Realm uncomfortably close to the sky...
 * The fairies in the Artemis Fowl series fled underground in order to hide from humans, and while not exactly thrilled to be there, they've made quite a society nonetheless.
 * Khazad-Dum, in The Lord of the Rings is beneath Middle-Earth.
 * The Tunnels series takes place almost entirely underground. Starting with the discovery of an underground city called the Colony, the protagonists move progressively deeper with each book.
 * In the fourth book in the Oz series, Dorothy and the Wizard in Oz, Dorothy is reunited with the Wizard, and they make a journey through various realms beneath the Earth, before coming back to the surface in Oz.
 * In Mikhail Ahmanov's Ash, humanity's only extrasolar colony is nuked in a War Of Earthly Aggression, resulting in survivors hiding in an underground city built just prior to the destruction. Other survivors live in a base under the ocean. Earth forces have no idea anyone survived but continue to nuke the surface. The underground survivors are building nukes to retaliate against the enemy base.
 * In Shadowmarch series, Funderlings designed and built entire town beneath the Southmarch castle, as well as labyrinth of caverns and tunnels beneath it. Similar subsurface Funderling town exist beneath Syan, and there are probably other beneath various cities through Eion.
 * The novel Dark Cities Underground posits that this trope exists as modern-day fiction's retelling of the myth of Osiris.
 * The Darke Halls in Septimus Heap are basically a huge, subterranean cavern that forms a Darke realm.
 * The Underland Chronicles is all about this trope. More than 90 percent of the books take place in an entirely underground world beneath New York called the Underland.
 * Kiki Strike: Inside the Shadow City.
 * The Weirdstone of Brisingamen's svart-alfar live beneath the earth, and the protagonists have to spend some time spelunking through abandoned (human) mine workings as well.

Live Action TV
"Dr. Brennan: Every society has its bottom-dwellers, and every society fears its bottom-dwellers, because they are a symbol of what happens in that society if you fail."
 * Beauty and The Beast featured "the World Below", an elaborately gothic underground colony secretly constructed long ago and connected with the sewers and subways of New York.
 * Fruity, Matt and Chaka in MTV's Downtown venture into the subway tunnels in New York so Matt can tag the trains; while they wander around, they talk about all the crazy inhabitants they've heard tell of. Suggestions include: survivalists, mole people, giant snakes, and transit cops who stage fights.
 * The miniseries and novel Neverwhere (both by Neil Gaiman) mainly take place in an elaborate underground society beneath the streets of London. There's definite magic in the setting, though, and some hints that the lower chunks of the system aren't really connected to the "real" world.
 * In the Bones episode, "The Woman in the Tunnel," Brennan and Booth investigate the murder of a woman who was doing a documentary about an underground society under Washington D.C. Brennan finds the society fascinating.


 * The underground shelter in the Doctor Who story The Enemy of the World.
 * Also the Undercity of New New York in "Gridlock".
 * The Compass Killer on CSI New York was tracked to his hiding place in a forgotten underground apartment beneath a park in Queens, and pursued through adjoining storm drains and sewer tunnels.

Tabletop

 * A small-press RPG, Underworld, explored this trope heavily.
 * In the Shadowrun game, the Seattle Underground, a real-world complex of abandoned and buried buildings from the 19th century, has been colonized by orks and trolls as a cross between this trope and an ethnic neighborhood.
 * Thousands of years earlier in the same Verse, artificial underground cities called kaers provided shelter for Earthdawn's inhabitants during the time of the Horrors.
 * Skullport, located under the city of Waterdeep, is an example of this trope from the Forgotten Realms.

Video Games

 * In City of Heroes, the titular city has an impressively large, intricate, cavernous, and heavily-populated sewer system. If heading down there, expect zombies, evil cultists, devil-worshippers, Nazis (untill they got Retconned out, anyway) and home-brew cyborgs. If you head deeper into the Abandoned Sewer System, you'd better bring friends, since it's virtually covered with interdimensional mutants, hostile aliens, demons, ghosts, and the occasional tentacled superdimensional monster-of-the-week.
 * Not to mention the "lost city of Oranbega", archaic and arcane ruins teeming with the Circle of Thorns and their summoned behemoths which lies beneath even the sewers.
 * Underworld a relatively obscure RPG, had a setting and flavor similar to that of Neverwhere, but was set under New York City instead of London.
 * Metro 2033 features the post-apocalyptic variant, with a small civilization of humans Fighting for Survival in the depths of the Moscow underground railway after a devastating global nuclear war.
 * In Final Fantasy XII, after The Empire invades Rabanastre, all of the city's inhabitants are forced to live in the underground tunnels formerly used for storage of goods and merchandise except for the extremely wealthy people.
 * The Locust Horde in Gears of War lives underground. The day that they began their war against humanity is referred to as "Emergence Day" due to them bursting up out of the ground.
 * In order to get just about anything done in Minecraft, you need to create labyrinthine tunnels deep underground while mining. You can stumble upon underground lakes, and deep caverns, and there are lots of monsters below looking for you.

Webcomics

 * In Sluggy Freelance, the tunnels beneath Kesandru House (or where it once stood) were originally just a squat stretch of bare earth that Bun-Bun hung out in. But then the Dig-Bots (tiny, self-replicating robots designed to improve the living space) got to work. Eventually they built their own night club down there, and even their own mall.

Western Animation
"Moleman: There is no escape from the Kingdom of the Moles! [Otto/Homer's bungie-cord retracts, pulling them upward] Moleman: Well, except that."
 * Futurama took on this trope, too, with a civilization of mutants living in the sewers of New New York. However, Fridge Logic can make one wonder why they don't use the ruined buildings rather than make their own junkheaps out of things flushed down the toilet. Maybe it's the running water, or maybe the writers felt that the Rule of Funny justified the contrast.
 * It is also rumored that there's a freakish race of sub-mutants living in the sewers of Old New York (under the sewers of New New York), but that's just a sub-urban legend.
 * A group of homeless underground people, with feline-humanoid friends, were featured in Gargoyles. They took over a secret lab built and later abandoned by a Mega Corp.
 * Parodied in The Simpsons episode "Hello Gutter, Hello Fadder", in which the sewers beneath Springfield contain Morlocks, C.H.U.D.s and Mole People (led, of course, by Hans Moleman).

"Homer: ...and that's when the C.H.U.D.s came at me. Marge: Of course you'll have a bad impression of New York if you only focus on the pimps and the C.H.U.D.s."
 * Also when Homer told the story of his first trip to New York, which culminated in him falling down an open manhole.


 * The cat gang in An American Tail.
 * Also, the exiled tribe of Native American mice in the third film (though this also crosses over with caverns, being that there is a system of small mouse-sized caves beneath the New York subway system in which the tribe dwells).
 * The Teenage Mutant Ninja Turtles live in New York's sewer system.

Real Life

 * In case you thought it was all fiction, check this.
 * There were so many catacombs, caverns, tunnels and quarries dug out beneath Rome and Paris that the ground literally is like swiss cheese. (buildings began falling into the ground) and had to be filled in with artfully stacked rubble made of the bones of millions of plague victims. Rome's catacombs were reputedly inhabited by persecuted Christians. Easy access to limestone bedrock, which could be quarried and burned to produce mortar and fuel, is probably why these cities grew so large in ancient times.
 * Watch Cities of the Underground on History channel.
 * Many, many people live underground in Paris, Rome, New York, and possibly other major cities. They range from the occasional traveler (mostly the case in Paris) to the homeless (the case in New York)
 * In 2003, the French police found a 400 M cinema in Paris's underground, fully equiped with electricity, phone, and even a small restaurant. The Parisian police even have a dedicated squad patrolling the underground to chase intruders.
 * In 1934, G. Warren Shufelt, working off of a Hopi legend of the Serpent Brothers, went in search of the lost underground city of the lizard people, tunneling a shaft down a thousand feet in downtown LA before eventually giving up.
 * The town of Coober Pedy, Australia. The only reason it exists is because opal was discovered there. Diuring the summer, surface temperatures can reach 50 Degrees centigrade, so the 2000-some residents all live in (quite luxurious) underground houses.
 * The ancient city of Petra, Jordan, carved out of rock. It was recently named one of the New 7 Wonders of the World.
 * Note that most of these 'buildings' are only façades. It is thought that the carved out buildings are either tombs, temples, offices etc. Most of the daily activities were going on in the camp on the plain between the mountains.

Anime and Manga

 * Subterranean civilizations armed with giant Robeasts are perhaps the second favorite Super Robot series antagonist after Alien Invaders. One of the first examples is the Dinosaur Empire of the Getter Robo series.
 * Followed only a few months later by the Mycenae Empire in Great Mazinger.
 * In a variation, Elemental Lord Cybuster has an entire world in the middle of the Earth, though this might be a magical dimension.
 * Naturally, this is based of the world of La Gias in the Super Robot Wars series
 * Tengen Toppa Gurren Lagann has a DIY version, with caves dug out.

Comic Books

 * Common in comics of yesteryear. Superman, Spider Man, The Fantastic Four, etc. have all had to deal with angry molemen.
 * Amulet: Sort of; it's implied that Alledia is actually a sort of alternate Earth. But they definitely get there by going down a very, very long staircase.
 * The Terry Fermians in the Carl Barks story "Land Beneath the Ground".

Film

 * An uncharted cave in The Descent hosts a group of Gollum-like, blind, stunted little cave people with remarkable agility and bat-like sonar capabilities.
 * It's also the main realm of The City of Ember.

Literature

 * In More Information Than You Require, the Mole Men live here with thir hideous steeds and the Trogloditic men. Fortunately for mankind, the Mole Men are relatively nice 18th Century Enlightenment-types
 * In the 1930 short story "The Mound" written by Zealia Bishop (but ghost-written by none other than H.P. Lovecraft) a Spanish conquistador descends into a blue-litten underground world of caverns called K'n-yan. In true Lovecraftian fashion, this world proves only the first of a succession of otherworldly abysses. Below K'n-yan is the grotesque red-litten world of Yoth and below that is the nightmare realm of un-litten N'Kai.
 * The Pellucidar novels of Edgar Rice Burroughs also qualify, with one big "cavern" consisting of the entire core of the Earth, complete with an internal miniature sun! Tarzan even went down for a visit in one installment.
 * In David Eddings' The Belgariad, the Ulgos have lived in the caves below Ulgoland for centuries. They are featured particularly in Magician's Gambit.
 * Haven City, home of the fairy folk in the Artemis Fowl series.
 * Given the quantity of Phlebotinium to which the Fey have access, Haven City is really more an Elaborate Underground Base than a simple Beneath The Earth dwelling.
 * Underland in CS Lewis's The Silver Chair, the fourth (or sixth, depending whether you go chronologically or by date published) book of The Chronicles of Narnia. However, the topmost layer is all the protagonists get to visit; turns out there is a world much much further down below called the Land of Bism.
 * The Tunnels series. Although ostensibly children's novels, the series is surprisingly dark as well as surprisingly good, and is set almost entirely in a Crapsack World which lies beneath our own.
 * Jules Verne's Journey to the Center of the Earth.
 * Tad Williams' Memory, Sorrow, and Thorn series heavily utilizes this trope. Not only is the Hayholt, capital of Erkynland, built atop the ruins of the Sithi city of Asu'a, but the entire continent of Osten Ard seems to be riddled with caverns and tunnels, the majority of which were constructed or at least "tamed" by the Dwarrows over countless eons. The Dwarrows use these tunnels to travel from one ancient city to another while remaining completely unobserved by mortals and Sithi alike. The tunnels beneath the Hayholt in particular are a major plot device - The Hero Simon gets lost there no less than three times and they are used by various factions to bypass the castle walls during the final battle. Further, they serve as a metaphor for Simon's Character Development.
 * Tailchaser'sSong has Vastnir,
 * Every dwarf ever. The most famous modern example (and the one most other portrayals are largely based on) is JRR Tolkien's The Hobbit.
 * The Descent and Deeper are novels dealing with the revelation that our entire Earth is riddled with this trope, and that cannibalistic near-human creatures inhabit the global maze of tunnels.
 * The Age of Fire series has the Lower World, a network of tunnels and cave systems said to be home to most of the surviving dragons. It is often mentioned during the first two books but only rarely and briefly explored. In contrast, most of the third takes place there; it turns out that the Lower World is not only absolutely massive, but may well be just as populous as the Upper.
 * Lovecraft lives on this trope. Pick a story, pick any story, 9 times out of 10 there's some chance that the "long-lost subterranean civilization of awkward syllables" will be mentioned.
 * The animals in Roald Dahl's Fantastic Mr. Fox build a labyrinth of caves and tunnels to live in and steal food from the rather nasty human villians.
 * The kingdom of Subterranea in the Doc Savage novel Murder Melody (although it takes the heroes about half the novel to actually get there).
 * Edgar Rice Burroughs wrote a series of novels about Pellucidar, a vast land deep underground inhabited with primitive humans and dinosaurs.

Live Action TV

 * All the Fraggles of Fraggle Rock lived in an elaborate cave hidden behind Doc's workshop. Or beneath the lighthouse, in the UK version. In both cases, exactly where the Gorgs' garden was in relation to the human world was completely unexplored.
 * In The Young Ones episode "Boring", our heroes are all bored out of their skulls. Meanwhile, just below their house, the king of the underground kingdom where nothing boring ever happens laments the fact that he may never meet a totally boring person.
 * An inversion occurs in the Farscape episode "Taking the Stone". Our heroes land on a "royal cemetery planet" where the surface is covered in the graves of deceased royalty and a society of young hippies live in underground caverns. Moya's crew discovers that the inhabitants are suffering from the effects of radiation which is amplified by the caves. Unlike most examples, the safer thing to do is return to the surface, even with all the graves...
 * It's not well explored, but Hollow Earth in Sanctuary fits here.
 * The Silurian city in the Doctor Who two-parter "The Hungry Earth"/"Cold Blood", which lay in a cavern many miles beneath rural Wales.

Music

 * The 1974 concept album The Lamb Lies Down on Broadway by Genesis (back when they were a prog-rock band) tells the story of a teenage street punk named Rael who gets sucked into a surreal world beneath the streets of New York City, where he encounters a variety of bizarre inhabitants and situations.
 * Tom Waits' wonderfully disturbing song "Underground" (A quote from which tops this very page) is all about this.

Mythology

 * Older Than Print: In both the ancient Norse and Celtic mythologies, certain supernatural folk lived underground or within and beneath hills and mountains.
 * We'll call it "mythology" for lack of a better term...during the late 19th Century and early 20th, a number of people became convinced that the Earth was hollow, and inside there was a lush paradise of eight-foot-tall blonde genius-poets who were, in many cases, hermaphrodites, usually supposed to be the perfect ancestors of humans. John Symmes even got some support from Congress to go look around the North Pole for an entry-hole. (John Hodgman covered this in his second book, but he wasn't as far off as you might expect.)

Newspaper Comics

 * Flash Gordon has the Cavern World of Syk, home of the Blue Magic Kingdom ruled by the Witch Queen Azura. Of course, that's beneath Mongo, not Earth.
 * Prince Valiant has the subterranean realm of the Dawn People. The current storyline has Val traveling through it to rescue Aleta, encountering monsters and what-not, in something very reminiscent of an old-school D&D dungeon crawl.

Tabletop Games

 * A subterranean world is a common fixture throughout Dungeons and Dragons. In Forgotten Realms and Greyhawk, it's called the Underdark, and both use it in more or less the same way—a very hostile environment filled with ancient evil races where Everything Is Trying to Kill You.
 * In 4th edition, legend has it that it was carved most large tunnels in it were out by the blind, agonized strugglings of Torog, God of Imprisonment and Torture, to escape after he was entombed alive during the Dawn War against the Primordials, smashing through the dimensional walls between the mortal world, the Feywild and the Shadowfell in his efforts.
 * In Eberron, the underworld is called Khyber after the primordial dragon-god from whose body it was purportedly created. Khyber is a world of pitch-blackness, ruled by Eldritch Abominations from Xoriat, the realm of madness. Cults of surface-dwellers worship the mutagenic energies that emanate from its black crystals.
 * In Pathfinder, the world beneath is known as the Darklands and draws inspiration from pulp fiction of the early 1900s, such as Edgar Rice Burroughs's Pellucidar. It's divided into three "layers"—the uppermost is home to fairly normal humanoids like goblins and dwarves, the middle layer is home to more reclusive races like the dark elves, and the deepest pits are inhabited by unspeakable horrors.
 * Ravenloft didn't have an underworld that spanned the entire setting, but one of the islands in the Mists was built on taking this trope to its most horrible conclusion—Bluetspur, home of the mind flayers.
 * Also, the Arak lived in a Beneath the Earth environment before the Shadow Rift appeared, while the domain of Timor relocated to lie underneath that of Paridon.
 * The Known World (later renamed Mystara) took this the whole nine yards and made the world hollow, with sub-setting called the Hollow World (as opposed to "Known"). The planet's crust between the Hollow World and the surface also had the typical cavernous underworld, inhabited mainly by the dwarves and the shadow elves (and nasty monsters, of course).
 * The Subterran Uprising from Monsterpocalypse.
 * The Skaven in Warhammer Fantasy Battle have a vast "under-empire" centered under the ruins of Skavenblight, with tunnels stretching all over the continent.
 * The Mountain Folk in Exalted live Underground in a Forever War against the Darkbroods, monstous races inhabiting even deeper tunnels far below the Flat World. This was caused by the Solar Exalted, who grew jealous of them, and forced Autochthon (their maker) to geas them into staying underground unless an Exalted said they could come up. Autochthon had the appropriate response to this, and left to Elsewhere.

Theme Parks

 * The Journey to the Center of the Earth attraction at Tokyo DisneySea is based around this concept.

Toys
"Lewa: Has anyone noticed how every time we go underground something bad happens?"
 * Onu-Koro and Mangaia in Bionicle.


 * There's also

Video Games

 * The RTS Cave Wars. True to form, your resource-management includes ensuring a good mushroom-harvest...
 * Wherever you don't find sewers in City of Heroes, you'll find caverns, with entrances in every park and abandoned lot. The caves tend to house either trolls (former humans mutated by exposure to a drug that induces super powers), beings of living stone, or the various groups of evil magicians. Occasional inter-dimensional aliens may be spotted, usually busy redecorating the place into an Elaborate Underground Base.
 * Lampshaded at least once in the intro text to a mission, describing your character wondering how any of the buildings in the city keep standing when the ground below is like Swiss cheese.
 * In Deadly Rooms of Death, the great underground empire is actually more advanced than the simple agricultural society above.
 * The RPG series Exile (and its remake, Avernum) takes place in a gargantuan network of underground caverns, stretching for several kilometers in every direction, and varying from huge caves big enough to have their own weather-systems, to twisting little mazes of passages, all alike. As the name of the area suggests, the people who live there didn't CHOOSE to do so, and sure enough, the place is littered with fungus, lava, and plenty of gloom.
 * Many towns on the surface of the world also have monster inhabited sewers as well.
 * Final Fantasy IV contains an enormous, open-space underworld, populated by dwarves.
 * The Locust from Gears of War live in a vast series of underground caverns known as The Hollow.
 * The Myst series of PC games gradually reveals that the lost city of D'ni is in a five-mile-wide cavern somewhere beneath a vast stretch of desert, in either Egypt or New Mexico (Cyan Worlds "claims" that, since the Myst saga is based on a true story and a real archeological find, conscience dictates that they keep the location a secret). One game in the series actually permits a virtual exploration of D'ni itself.
 * New Mexico is canonical - the sign near the property entrance in Uru identifies the specific county. The Middle East answer is from the author of the novelization, who *ahem* wasn't told a whole lot because the Miller brothers hadn't decided yet.
 * Warcraft games have their share of underground caverns. Azol-Nerub, the capital of the Nerubian civilization, is a sprawling underground labyrinth that stretches across much of the continent of Northrend. The part where the Nerubians used to live is called the Upper Kingdom, and below that is the Lower Kingdom, which consists of even deeper tunnels populated by nameless horrors and possibly one of the Old Gods. The Old God has been confirmed to be Yogg-Saron in the Wrath of the Lich King expansion.
 * The first 40-man raid instance in World of Warcraft, Molten Core, is a huge lava-filled cavern located deep under Blackrock Mountain. It's home to elementals, lava giants, core hounds and Ragnaros, the elemental lord of fire.
 * And of course there's the Undercity, the vast subterranean home base of the Forsaken, built underneath the ruins of another city.
 * The Sierra adventure game Torin's Passage has no less than four separate worlds Beneath the Earth, each one nested inside the other. In the outermost world, being banished to The Lands Below is a form of capital punishment.
 * The original Zork text-based game features exploration of the Great Underground Empire.
 * Dwarf Fortress has implemented no fewer than three levels of underground cavern system, each progressively nastier than the last, Dig down past all three of those, and you'll find a great sea of magma and "semi-molten rock".
 * Master of Orion 2 has the Subterranean racial trait. Being a tunneling sort means that your colonies can support a greater population, and ten points are added to to trooper effectiveness when defending against a ground invasion.
 * The underground in Touhou has its own city, and is also stated to be the former location of hell before it was moved.
 * Age of Wonders has two map layers (until Shadow Magic added the third), with caves allowing access to the Underground level.
 * Baldurs Gate II has the Underdark, a vast subterranean labyrinth covering much of the world.
 * Dragon Age has Orzammar, city of the dwarves, and the Deep Roads, which once linked dwarf cities but have now been overrun by darkspawn.
 * Several instances from Zelda:
 * The Legend of Zelda Oracle Games feature the Subrosians, who live in the realm of Subrosia right underneath Holodrum.
 * The Legend of Zelda Skyward Sword's backstory explains how The Dark Times in Hyrule began when a great army of demons emerged from the earth and ravaged the land in their campaign to obtain the Triforce.
 * Videogame/Quake has a secret level named "The Underearth".
 * Dark Souls has the player spend a lot of time in a variety of underground areas, such as demon infested lava ruins and the Tomb of the Giants.
 * Dark Souls has the player spend a lot of time in a variety of underground areas, such as demon infested lava ruins and the Tomb of the Giants.

Webcomics
""The valley was actually several hundred miles from the center of the Earth, but any mole person suggesting the surface did not revolve around them was fed to dinosaurs.""
 * From Sluggy Freelance:
 * Most of what is seen of the Dimension of Pain seems to consist of caverns; the mostly canonical "Meanwhile in the Dimension of Pain" guest strips confirm that there is a surface above them.
 * The Valley That Time Forgot In The Center Of The Earth!!!

"Lars: ...how big can these tunnels be? Maxim (ex-cavalry man jager): How big?! Ho ho! [...] it took us two years to get outta dose caverns."
 * Girl Genius has Deepdown. It's big.

Western Animation

 * In Avatar: The Last Airbender, the metropolis Earth Kingdom capitol of Ba Sing Se was once a subterranean city that dwelled within crystal catacombs carved out by Earthbenders. Following expansion, the caverns were abandoned and the lost city is now know as Old Ba Sing Se.
 * The Fire Nation Capital (built in the corona of a dormant volcano) also has a series of underground tunnels. Mainly used as a defense bunker in case of emergencies.
 * The leprechauns in the Chip 'n Dale Rescue Rangers episodes live in underground places, too, which in the case of the leprechaun king Darby Spree can be pretty spacey.
 * Parodied by The Underminer at the end of The Incredibles: "I am always beneath you, but nothing is beneath me!"
 * My Little Pony has the Dell Dwellers from "Mish Mash Melee" and the Kingdom of the Lava Demons from "Quest of the Princess Ponies".
 * My Little Pony Friendship Is Magic has the Diamond Dogs from "A Dog and Pony Show", a group of subterranean-dwelling dog-men.
 * The Mole People featured in The Tick (animation) are actually a peaceful but weird group who didn't want to conquer the surface world but to just take a vacation. The Lava men on the other hand...
 * Ned's Newt has trolls living beneath the Earth's surface, who sometimes try to Take Over the World, only to be stymied by the protagonists.