Darkening Depths

Darkening Depths is a mod that adds new cave biomes to Minecraft as well as some new ores and two new dimensions.

Stony Forest: This is an underground forest made of trees that have turned to stone. It also has a new block, Mossy Stone, and the new Stone Wood. 

Mushroom Caves: This is a mushroom biome that has some new mushrooms; such as blue mushrooms, green mushrooms, stone mushrooms, nightcap mushrooms, and spidershrooms.

Icy Caves: This is an underground Ice biome, with a new mob called the Frostbite Spider, as well as a new Icy Ore.

Desert Caves: This underground cave biome is mostly made out of sand and sandstone, with some red sand as well. There are two new mobs here; the Sandcrawler (a new spider) and the Sandy Creeper. You can also find a new Desert Ore here.

Nether Caves: This biome adds the Nether to Minecraft's caves. Here you can find a new Flaming Ore, as well as some magma cubes and zombified piglins. If you're lucky you could even find a fortress or bastion.

End Caves: This biome is like the nether caves, except with end stuff. It also has the new Enderite Ore, which can be used to make powerful gear.

Burnt Forest: This is a surface biome composed of scorched dirt, as well as the new scorched trees. You can find a few structures here with somewhat decent loot.

The Below World: This is a new nether-like dimension where you can find all the cave biomes.

Jade Temple: This is a new structure filled with loot and deadly enemies.

New Bosses: This mod also adds some new bosses.

New Armors and Toolsets: There are 6 new Armor and Toolsets that all have various effects (except for desert and mushroom)