Cave Mouth

The mouth is like a cave. It's a breeding ground for bacteria, it's dark, and damp, too. Teeth are like the stalagmites and stalactites. Sometimes authors can't resist the Visual Pun of a literal cave mouth. In some variations, the cave is actually the mouth of some large living creature.

This can either be pure Eye Candy or relevant to the story.

Examples of Cave Mouth include:

Film

In Aladdin, the Cave Of Wonders' opening is a big tiger's mouth with sharp teeth. It also talks ("Who disturbs my slumber?").
- Aladdin and the King of Thieves had a huge turtle too whose mouth was like a cave.
In Disney's The Little Mermaid, the front door of Ursula's lair is the mouth of a sea-dragon skeleton.
The Empire Strikes Back had the crew of the Millennium Falcon confuse a Space Slug's mouth with an asteroid cave.
Monstro's mouth in Pinocchio.
In The Goonies, there is a skull-shaped cave entrance.
The Cave of the Beast in The Twelve Tasks of Asterix. Bonus points for closing after they enter.
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.

Literature

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.
- Discworld Trolls do this as they get older and, since they grow all their lives and are pretty much indestructible, bigger. Once a troll gets very old it decides to go to some remote area and have a 'think' about life. The thinking gets so intense, it leaves the troll in a semi-dormant nearly-dead state of slumber (they aren't that bright you see), meaning that many of the hills and mountains on the Disk are actually trolls that went off for the 'big think' and have stayed laying there for what is sometimes millennia. This is also the cause of much of the historical conflicts between Dwarfs and Trolls, Dwarfs being small humanoid creatures that smash open rocks to extract valuable minerals and Trolls basically being large walking rocks with valuable minerals inside. As the book says, it is hard to have decent relations with someone who just 'quarried' your great-great-great-great-great-grandfather while he was out 'thinking'...
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 Warrior Cats series, the cats call a certain cave "Mothermouth" because it resembles a mouth.

Live-Action TV

In The Tenth 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 Fantaghiro 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 ^[1] 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."

Newspaper Comics

One comic of Conan the Barbarian had 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.
The Phantom lives in the Skull Cave, which is an obvious example of this trope.
A Dilbert comic had a cave with an entrance that resembled a somewhat different orifice. The previous comic did say that Asok had to travel to "the bowels of the bureaucracy"...

Tabletop Games

Card Games

The card Howling Mine from Magic: The Gathering looks like this most of the time, Depending on the Artist.

Tabletop RPG

Call of Cthulhu (tabletop game) 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.

Theme Parks

The Storybook Land boat ride at Disneyland begins by entering Monstro the whale from Pinocchio.

Toys

Bionicle: the entrance to Roxtus, which isn't surprising as it's the head of a Prototype Robot.

Video Games

Super Mario Bros. 2, anyone? Every level's exit is a Hawkmouth, 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 Brutal 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.

Web Comics

Pictured above, in Erfworld, the entrance to the caves leading to Gobwin Knob has a sinister resemblance to a mouth.

Web Original

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.

Western Animation

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 and 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.
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.
"BETTER PLAY SAFE!!!"

Real Life

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.

↑ uvula

[1] uvula

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