Cavebiomeapi Mod
CaveBiomeAPI Mod (1.17.1, 1.16.5) allows modders to add actual underground biomes to give more life to the depths of our beloved worlds. The underground has never been a very interesting zone to explore to be honest.

Screenshots:

Dripstone Caves generating next to Lush Caves

Azalea Tree generating on Tundra Biome

Compatible with Official Datapack
Requires:
How to install:
How To Download & Install Mods with Minecraft Forge
How To Download & Install Fabric Mods
CaveBiomeAPI Mod (1.17.1, 1.16.5) Download Links
For Minecraft 1.15.2
Forge version: Download from Server 1
For Minecraft 1.16.5
Forge version: Download from Server 1
Fabric version: Download from Server 1
For Minecraft 1.17.1
Forge version: Download from Server 1
Fabric version: Download from Server 1