There's a good reason the entrance to any underground cavern is called a "mouth".
Consider that the mouth is a breeding ground for bacterianote ; it's dark, and damp, too, and has a lot of sharp objects lining its roof and floor.
Now consider a typical cave: Dark, possibly damp, a breeding ground for all sorts of unsavory creatures, with stalagmites on the floor and sharp (and occasionally falling) stalactites lining the ceiling...
So is it any wonder that sometimes authors can't resist making a Visual Pun out of the "mouth" to a cave?
Sometimes this goes beyond the visual metaphor and actually becomes a plot-relevant — perhaps That's No Cave.
- One comic of Conan the Barbarian has a rather literal example of this. In his efforts to steal two strangely eye-like (yet angled) gems from a greedy merchant, Conan and the merchant both go chasing after the gems when they start flying through the air, leading them through a strangely mouth-like cave (with irregular sets of "teeth" set the whole way through) to a large central chamber that is covered with deep piles of gemstones. The merchant immediately dives into the mixed array of rubies, diamonds, sapphires, emeralds and other gems, noting that it's extremely strange to see so many different types of gem in the same place, but too blinded by greed to care. Conan is more cautious, and thusly he survives when it turns out they're in the stomach/throat of some kind of mountainous earth elemental. The merchant is devoured.
- Tales of Telguuth: A brother and sister who are about to be offered as the final sacrifices required to resurrect an evil god run away from the priests and take shelter in a cave in the desert. When the brother climbs out it turns out that the cave was really the mouth of the god, who eats the sister so it can rise from its slumber and lay waste to the world.
- In Aladdin, the Cave of Wonders' opening is a big tiger's mouth with sharp teeth. It also talks. ("Who disturbs my slumber?")
- In Disney's The Little Mermaid, the front door of Ursula's lair is the mouth of a sea-dragon skeleton.
- Monstro's mouth in Pinocchio.
- The Cave of the Beast in The Twelve Tasks of Asterix. Bonus points for closing after they enter.
- In The Brave Little Toaster, the gang of appliances is seeking shelter and Lampy discovers a hollow tree. He hops in, calls to his friends, then turns on his light. From the outside, the illumination creates a scary face like a jack-o'-lantern, and everyone else screams and vanishes into the bushes. Lampy can't see it because he's inside the tree itself, once he figures it out, hilarity ensues.
- In The Pagemaster, our heroes find a cave, which is full of stalagmites and stalactites. When Adventure accidentally breaks one, it's revealed that the "cave" is actually the mouth of a dragon.
- Drake's island from The Pebble and the Penguin, which for some reason looks like it should've suited Frankenberry more than an evil penguin.
- In the eleventh Give Yourself Goosebumps book, Deep in the Jungle of Doom, one path has you entering a cave that is strangely damp, with soft pink walls. It turns out you're in the mouth of a giant rock monster, and your friend let it eat you in exchange for her own life. Naturally, this is a Bad Ending, and ends with you being Swallowed Whole and waiting for death as you plunge towards the stomach...
- In The Catastrophe of the Emerald Queen. The entrance to The Shores.
- In The Light Fantastic Twoflower's kidnappers inadvertently settle in the mouth of an enormous troll, mistaking it for a cave. They probably would have been fine if they hadn't lit a fire inside it, ironically to ward off trolls.
- In The Seventh Tower cavernmouths look like regular caves... except for gleaming red dots. These are tonsils, and if you stray too close to the "entrance"... the jaws come out.
- The gate to the Colony in the Tunnels books is called the Skull Gate, because it rests below an enormous stone skull.
- In the The Spriggan Mirror, the castle- presumably Derathon's- was said to have the only entrance carved like a gaping mouth.
- In the Warrior Cats series, the cats call a certain cave "Mothermouth" because it resembles a mouth.
- In The 10th Kingdom, the cave entrance is not just shaped like a dragon's head, it's an actual dragon's head. A dead, fossilized dragon (hence the name "Dragon Mountain"). This introduces a bit of Squick when the cast must take mining sleds down along the spine and ribs toward the actual mine and get tossed out the other end.
- In the Star Trek: The Original Series episode "The Apple", you've got the mouth of Vaal, a cave mouth shaped like a dragon's head, complete with glowing eyes.
- The Cave of the Golden Rose from Fantaghirò is actually a monster's mouth, and the rose is its uvula. The cave even gave its name to most translations.
- The Goodies go caving. They find a remarkable Cave Mouth. They note the curving rows of stalagmites and stalactites and treat the odd red thing note like a punching bag. The giant dinosaur, naturally, closes its mouth on them.
- Doctor Who. In The Face of Evil a giant face of the Doctor has been carved into the cliff by the mad computer. The Doctor indignantly rejects the idea that they enter the cave by climbing up 'his' nostrils. "It's up over the tongue and down the throat."
- Call of Cthulhu supplement Fragments of Fear, adventure "Valley of the Four Shrines". The entrance to the cave that leads to the valley has been carved in the likeness of Cthulhu's head.
- The Storybook Land boat ride at Disneyland begins by entering Monstro the whale from Pinocchio.
- The queue line for Skull Island: Reign of Kong at Universal's Islands of Adventure goes through several cave caverns, which have mouth-like archways scattered throughout.
- Great America's roller coaster The Demon has featured a high-speed plummet into a Cave Mouth since its theming in 1980.
- Super Mario Bros.
- In Super Mario Bros. 2, every level's exit is a Mask Gate, shaped like a huge bird head.
- Also, the map icon for "Bowser's Galaxy Generator", the final level of Super Mario Galaxy 2, which is actually shaped like Bowser's head.
- As with the map icons for the first two Bowser levels in the original Super Mario Galaxy. And the fish planet in Bigmouth Galaxy.
- In Quest for Glory IV, the cave you start in the beginning. It turns out that it's an Eldritch Abomination.
- The entrance to the Sea of Black Tears in Brütal Legend is shaped like a giant skull.
- Grunty's lair from Banjo-Kazooie.
- The entrance to a cave in Final Fantasy Adventure not only looks like a mouth, it has a sense of taste as well, and will spit you back out if your armor isn't tasty enough (it likes silver).
- One level of Golden Axe 2 takes place in "the cavern called Dragon's Throat". Guess what the entrance looks like?
- In Dark Fall: Lost Souls, Amy's creepy drawing of Dowerton Station depicts the entrance to the train tunnels like this.
- Almost all the dungeon entrances in The Legend of Zelda consist of these. The Legend of Zelda: Skyward Sword also features a couple as a Call-Back to the first game.
- El Diablo's cave in Desperados, as a skull.
- A giant monkey head serves as the entrance to the caverns of meat in The Secret of Monkey Island. The head itself is attached to a giant skeleton buried underground...
- The Tyrano Lair in Chrono Trigger features doors styled after theropod maws which open and close depending on switch configuration.
- Two entrances to the Annihilator Factory in Hero Core have a mouth design.
- The cave entrance to the Oubliette of Suffering in Shantae and the Pirate's Curse is one with its "teeth" initially blocking the way. It will only open its mouth to stinky ne'er-do-wells.
- The cave in which Lewis was murdered in Mystery Skulls Animated had an eerie mouth like opening with four holes above it that looked like nostrils and glowing eyes due to the green entity in the cave.
- Felarya is another fictional setting where it is rarely wise to explore such cavern mouths, since the ambush predators tend to be of sufficient size.
- In SpongeBob SquarePants, Sandy goes into what she thinks is a big pink cave and punches out what she thinks is the Alaskan Bull Worm. She had actually walked straight into its mouth and beat up its tongue.
- In Mighty Max, the cave entrance is not just a dragon's head, it's an actual dragon's head.
- Done in "In Search of the Dungeon Master", an episode of Dungeons & Dragons.
- In a Looney Tunes short called "The Mouse That Jack Built", a cat disguises itself as "The Kit-Kat Club," a night club with a cat-shaped entrance. Its tongue provides the red carpet.
- The entrance to Hell seen in Pluto's Judgement Day is a large, cat-shaped cave (which coincidentally resembled the entrance to the Cave of Wonders seen in Aladdin many decades later) in the middle of a volcanic area.
- An episode of Dragon Tales had a giant cave turn out to be the mouth of a giant rhinoceros (who also had a Ribcage Stomach).
- Chase Young's lair from Xiaolin Showdown.
- In "Not Very Nice" of The Trap Door, Berk drops one of the Thing Upstairs' eyeballs down the Trap Door and goes down looking for it. He finds it in a cave that turns out to the mouth of one very large creature;
Berk: Yech! There's that horrible smell again! Hang on, there's sommit funny about this place. The walls are slimy, and that green thing looks like a tongue, and them things over there look like teeth... Ohhh, globbits! I'm inside the mouth of some horrible thing!
- A chamber in Virginia's Luray Caverns has an entrance like this.
- Some ambush predators can do this to smaller, not-too-clever prey such as bait fish (For example, the alligator snapping turtle for one thing).
- The museum in Copan, Honduras (a major Mayan site) has an artificial mask-style carving of a head for an entrance.
- Be honest, this is how you ate Teddy Grahams or Goldfish as a kid.
- The old Sri Lankan royal fortress was called the Lion's Mouth, because it could only be accessed through a gate that was carved to look like the gaping mouth of a lion. Enormous paws were carved below the gate. These paws still stand today, while the mouth is gone.
- The entryway to Gatorland in Florida is meant to look like a giant alligator mouth.
- One defunct restaurant chain called "Coon Chicken Inn" (which got a namedrop in C.S.A.: The Confederate States of America) designed its entrances to resemble the mouth of a smiling African American porter, represented in stereotypical blackface style.