Darker Depths is a underground expansion mod.

There are currently 3 biomes added to Darker Depths:

Glowshroom Forest

The glowshroom cavern is where you will find large glowshrooms, the glowshroom monster, glowgrime, and the Glowshroom Cap.

The Glowshroom Cap when worn, has limited durability, provides light, and also provides the user haste 1.

Sandy Catacombs

The Sandy Catacombs is a desolate wasteland. Petrified roots hang from the ceiling, aridrock & boulders dot the inner cave landscape.

A new class of mob without souls can be found here. They wander the catacombs in the hopes of binding with their next victim.

Void Soul mobs when killed, don't die, but rather are turned into Void Souls. This orb is invulnerable, but will expire after 60 seconds. Upon right clicking with a glass bottle, Void Souls can be captured into a jar. Right clicking with Amber stops them from expiring.

A new structure is found here, aptly named the Catacombs, where you will find Body Snatchers, Void Soul Knights, Tombs, Paranoia, and Forsaken Bronze.

Cursed by the presence of the Void Souls, this Forsaken Bronze allows for unnatural abilities. This metal can be used to craft a few useful items, including the Death Anchor, the Stiletto, and Tombs.

Molten Cavern

The Molten Cavern is a raging hot biome, where Deepslate has turned into Darkslate and lava can be found in layered pools. Geysers, Amber Clusters, and Dead Living Crystals can be found here.

Dead Living Crystals can be reawakened by giving them a diamond. From there, they will start to grow back to life. When they have fully grown, they will turn into Living Crystal, which can spread to nearby diamond ore, or grow a taste for Melons, if you give them enough time.

Crystal Melons are formed when a Living Crystal has a hunger for Melons. Placing a Melon near a living crystal will cause it to turn into a stone melon, then growing inside until it finally breaks free of its shell, converting to a Crystal Melon.

Crystal Melons when used while in the offhand, will supercharge any tool to make it unbreakable, give 20% more attack speed, mining speed, and attack damage, for 5 minutes.
(Adjustable via config)

Yes, 1.20.1 backport is coming for 2.1!

Wiki found here: Wiki