Contrast Underwater Index and Index in the Sky.
Despite the names, Noob Cave (the earliest, easiest level in a game) and Platonic Cave (named for Plato's allegory likening a deceptive artificial controlled "world" to being trapped in a cave) are not examples.
Tropes:
- Abandoned Mine: Abandoned mineshafts and mining complexes used as unnerving and foreboding locations, concealing unpleasant things in their shadows.
- Absurdly Spacious Sewer: Strangely roomy, extensive, traversable and well-lit sewer systems.
- Ancient Tomb: Often found underground.
- Apocalyptic Underground Refuge: People shelter underground to escape disasters on the surface.
- Beware the Skull Base: Giant skulls acting as dungeons or fortresses.
- Bunker Woman: A woman is kidnapped and held in a contained location (usually underground).
- Buried Alive: Being sealed underground while still alive.
- Cave Behind the Falls: A cave whose entrance is hidden by a waterfall.
- Cave Mouth: A cave whose entrance looks like a mouth.
- Claustrophobia: Caves often prompt feelings of being trapped.
- Creepy Basement: Rooms underground are dank and frightening.
- Creepy Cave: Natural caves are scary and dangerous.
- Crystal Landscape: Often found in caves.
- Dig Attack: Attacking an enemy by burrowing underneath them.
- Digging to China: Digging through the Earth to emerge at another point on its surface.
- Down the Drain: A level that takes place inside a sewer or other pipe system.
- Drill Mole: Moles tunneling with literal drills.
- Drill Tank: A weaponized vehicle that drills its way through the earth.
- Dug Too Deep: People who dig too deep into the earth risk encountering dangerous things indeed, deep underground.
- Elaborate Underground Base: Villains often set up shop in complex and high-tech bases hidden underground.
- Fantastic Underworld: A vast network of caves and abysses beneath the surface world, with wonders, horrors, and strange creatures.
- Fast Tunnelling: Characters who can dig unnaturally fast, or just unrealistically so.
- Funny Terrain Cross Section: Cross-sections of the soil showing all sorts of wacky and humorous things buried in the earth.
- Land Shark: When threatening sharks decide to swim in the ground instead of the sea.
- Minecart Madness: Mine level in videogames using the minecarts and their tracks as transportation.
- Mole Men: An underground civilization of vaguely humanoid creatures.
- Mole Miner: Funny Animal moles portrayed as miners.
- Mole Monster: A videogame enemy that burrows underground, emerging only to attack.
- Mystical Cave: A cave with magical or spiritual qualities.
- Our Dark Elves Are Different: The Norse Svartalfar lived beneath the earth, and many of their modern adaptations follow suit.
- Our Dwarves Are All the Same: Stereotypical dwarves live in underground halls mining the earth.
- Planetary Core Manipulation: Adjusting or draining the core of a planet.
- Rollercoaster Mine: Out of control minecarts on elaborate underground tracks.
- Sand Worm: Enormous, wormlike beasts that tunnel unseen underground to attack prey walking above.
- Secret Sewer Society: A society living secretly in the sewers, subways, or other spaces beneath an inhabited surface city.
- Secret Underground Passage: A subterranean passageway that acts as a way to get from point A to point B without being detected.
- Sinister Subway: Subways used as spooky or sinister settings.
- Stalactite Spite: Falling stalactites.
- Subterranean Sanity Failure: Being underground does nasty things to the mind.
- Subways Suck: Subways are more trouble than they're worth.
- Thrown Down a Well: Imprisoning someone someplace they cannot escape from, generally a pit like an oubliette.
- Time Capsule: A buried capsule with a record for those who dig it up.
- Tunnel King: A character who specializes in digging and/or inhabiting tunnels and mines.
- Tunnel Network: A complex system of tunnels allowing for stealthy or sheltered movement below the surface.
- Under City: A city that used to be on the surface, but is now deep underground.
- Underground Alien Civilization: An Alien society that lives completely underneath the surface of another planet or celestial body.
- Underground City: Inhabited, full-scale cities beneath the earth.
- Underground Level: Videogame levels set underground and inhabited by cave- or ground-themed enemies.
- Volcano Lair: A secret base inside a volcano.
- Well Entry: A water well is an entrance to an underground or underwater place.
- A Way Out of a Cave-In: The heroes seem trapped underground, but then they notice the telltale sign of a way out.
- Ye Olde Nuclear Silo: Former military construction, typically subterranean, that can be used as a base.