A world generation mod adding in nine new underground biomes:
the Frozen Caves, the Mushroom Caves, the Molten Caves, the Amethyst Caves, the Black Caves, the Arid Caves, the Floral Lush Caves, the White Caves, and the Painted Caves.

I was busy with college and just saw this again but 160 thousand downloads!? Thank you all so much! If anyone has any ideas for new cave biomes, feel free to suggest some! Profundis is a world generation mod which adds nine new vanilla-styled underground biomes to the game. These seven new underground biomes make the depths of the world significantly more diverse—what was once just three underground biomes is now ten! Mojang claimed that Caves & Cliffs would make the underground even more varied than the surface; Profundis aims to add much needed variation to the underground. Profundis is a server-side mod, meaning it can be installed on a server and clients can join without needing to install a mod, much like a plugin! It can also be installed on a client for singleplayer!

Biomes

❄The Frozen Caves typically generates below cold regions of the world, and has icicles hanging from the ceiling with small ice and snow patches on the ground. Aquifers generating here have a layer of ice on the top, just like frozen lakes and rivers on the surface. Redstone is also extrememly abundant here. 🍄The Mushroom Caves can generate beneath more humid biomes. They have mycelium patches on the ground, with a plethora of small red and brown mushrooms generating everywhere. Huge mushrooms also spawn, although they spawn more often in large caves than small caves. Shelf Fungi line the walls, occasionally glowing with Shroomlights, in addition to the biome's higher rate of Glow Lichen. 🌋The Molten Caves can generate in almost any environment. They have lava deltas meandering throughout the stony slopes of the caves, with an abundance of lava springs as well. Basalt columns similar to those in the Basalt Deltas also spawn, although not as frequent as in the Nether. Small patches of Smooth Basalt can occasionally generate on cave surfaces in the Deepslate layer as well. Iron is more abundant here. 🔮The Amethyst Caves generate underneath hillier areas. They are filled with beautiful Amethyst Veins, with calcite and smooth basalt cracks in the cave walls. The crystals leave a gentle glow, alluring miners with their serenity. ⚫The Black Caves generate in almost any environment. They have walls of Tuff, Deepslate, and Blackstone, hence the name. They also lack Glow Lichen, making it difficult to see in large caverns. 🟧The Arid Caves generate under hotter areas. They have floors of sand and red sand, with sand in higher levels and red sand below deepslate level. The walls are made up of Terracotta bands of a variety of colours. They generate much like Frozen Caves, but in hotter biomes and rarer. 🌸The Floral Lush Caves are a variant of Lush Caves. They are exactly identical, except with a floor made of a variety of flowers. Pink Pedals coat the moss, while a mix of all flowers native to the Flower Forest grow in the moss. ⚪️The White Caves generate in almost any environment. Much like the Black Caves they have walls of Diorite, Calcite, and Andesite, hence the name. They generate in the same area as the Black Caves. 🎨The Painted Caves generate most commonly in tropical areas. They have walls of multicolored terracotta in a mesmerizing rainbow pattern. All of the cave biomes' generation can be disabled individually in the config by simply changing a value.

Structures

🪵Cabins generate in every Vanilla and Profundis cave biome, but not in barren caves. They are small to mid-sized bunkers, originally constructed as small bases for miners. They each contain a few functional blocks such as furnaces and crafting tables, plus barrels with loot. The barrels typically have mediocre loot with a pickaxe thrown in, but occasionally contain more valuable treasure. In addition, the loot found in the barrels matches what biome the structure spawns in. 🧊Ice Dungeons are a variant on regular Monster Room dungeons that spawn in Frozen Caves. They contain an icy arena with a Skeleton Spawner and 8 pre-spawned Strays (I opted to not use a Stray Spawner as to not contradict with Vanilla's precedent on Strays; if you would like that to change let me know!). They additionally contain four barrels full of typical dungeon loot plus snow and ice. 🍄Mushroom Houses are an uncommon structure generating beneath Mushroom Fields and in Mushroom Caves. They are non-hostile and contain a few chests of loot, plus some standard utilities. Loot is not very rewarding, but can be a useful source of food in a pinch. 🔮Amethyst Dungeons are a slightly more difficult version of regular Monster Room Dungeons, generating in Amethyst Caves. They contain one Zombie spawner and one Skeleton spawner, and a Diamond Block in the middle as a reward. ⛲️Sanctuaries are underground fountains generating in Lush Caves and Floral Lush Caves. They contain a Gold Block and a rare blue Axolotl. 🏮Blackstone Lanterns are small lightposts generating in Black Caves. They contain one barrel in the center of the structure—but make sure to open the barrel at night for optimum loot! If opened during the day, their loot will be lackluster. 🗿Deep Statues are uncommon humanoid statues generating in Frozen Caves, Mushroom Caves, Amethyst Caves, Black Caves, and Arid Caves. They contain eight Iron Blocks in the head of the structure.
If any of you have any ideas for new cave biomes or structures please let me know on my GitHub! I'm always open to suggestions!

Frozen Caves
Mushroom Caves
Molten Caves
Amethyst Caves
Black Caves
Arid Caves
Floral Lush Caves
Cabin
Ice Dungeon
Mushroom House
Amethyst Dungeon
Sanctuary
Blackstone Lantern
Deep Statue

Profundis: Nine New Quality Cave Biomes