Caverealms by HeroOfTheWinds
Adds underground realms.
Cave mods
A collection by Opons
Every mod I see that is related to caves
Packages
Caverealms Lite by Shara
Adds underground realms.
Dwarf Fortress style caverns by FaceDeer
Adds vast underground caverns in the style of Dwarf Fortress, complete with underground flora in diverse biomes.
Everness by SaKeL
Adds many new biomes, items and blocks.
Karst Caverns by MisterE
A cavern generator that creates complex karst cavern systems, with rooms interconnected by multilevel tunnels. Experience the joy of caving.
Underground Challenge by Hypernoot
Explore all 62 underground biomes!
Utilities for Underch - extension pack by Hypernoot
Extend the Underground Challenge by miscelaneous utilities and change some recipes
Livingcaves by Liil
Adds more life to the caves of your world
Wilhelmines Living Caves Mobs by Liil
adds mobs to the caves of your world
Deepslate by Silver Sandstone
Adds deepslate and related building materials.
Dense Ores by benedict424
Adds rare ores with twice the drops of their normal counterpart.