Minecraft wiki entry for Cave, associated text: A cave in early Classic.
A cave in Classic 0.30.
A wormhole entrance.
A dead bush in a badlands cave.
A pit generated in a forest biome.
A cave with all seven types of overworld ore.
An entrance generated in a badlands biome.
A cave generated near the surface of a jungle biome, causing vines to grow on its wall.
Various ores.
A water cave, with water flowing inside.
The inside of a cave near the surface of badlands.
A cave that generated underneath an ocean.
Unusually repetitive cave generation seen in spectator mode.
A deep cave entrance.
Natural sandstone blocking sand from entering a cave system.
Dark cave entrance.
Mobs that spawned in a unlit cave.
Cave entrance in a desert biome.
A circular cave entrance generated in snowy plains biome.
Huge cave generated inside a hillside.
Pitch-black cave.
In desert players have to be careful about mining in the ceiling, this can cause cave-in and easily kill the player.
Another example of a mega cave with a mineshaft.