Cave Biomes requires the library Datapack Utils. You can find it's Planet Minecraft page here.

Redefine your Underground! Cave biomes adds 34 new randomly generated cave biomes, 6 new cave shapes, and 5 new structures to explore!

Current features include but are not limited to:

• Improved normal cave generation
• Overgrown caves
• Underground Forests for each wood type
• Ice Caves
• Sandstone Caves
• Terracotta caves
• Nether caves
• Some special secret caves you'll have to find on your own
• Underground villages
• New dungeons themed around the 4 classical elements: earth, air, fire, water
• Special custom items found within said dungeons: good luck finding them
• And so much more!

Note: this datapack will probably cause lag when generating new chunks. It will clear up though and will not affect gameplay when not generating new chunks.
Another Note: Take a backup before adding to an existing world. Generation applies retroactively and it might grief underground structures you've built!