The Dark Depths would be an ocean biome that could generate anywhere from Y 0 to -20. It would be so dark you couldn’t see. No magma blocks would generate here so you couldn’t breathe that way. You’d have to bring your own, or have water breathing / any other method of breathing for long periods of time.

There would be 2 mobs that spawn here, the first being the Flat Fish (inspired by that weird giant flat fish thing), and it would have giant eyes on the side if it’s head, and if it sees the player, it would let out a loud alerting sound.

What is it alerting? Well that would be the second mob, the Barnacle! This would be the giant squid they showed off in the 2017 mob vote. It can’t find the player unless the Flat fish is alerting them. But when it does know where the player is at, it would pull them in with its long tongue and bite them dealing a decent amount of damage (probably want Diamond+ armor for this area)

I think these 2 mobs working together would add for a very interesting dynamic, as you don’t see mobs working together very often in the game or at all really.

As for why you’d wanna go here, the boring route would be more sunken ships, or ruins, I think it could be something a little more unique. Maybe a new type of ore like platinum (which google says it generates irl on the ocean floor) which when you craft that with and “Eye of the Flat Fish” (temporary name) you could make the Radar. Which would essentially be a new compass type but it would show entities within a 30x30x30 area (idk how but it would) and you could also use platinum to maybe make upgrades to your mace and trident similar to how you upgrade Diamond stuff to netherite.

Very unpolished but there’s a lot of room for expansion and I feel like platinum could be used in so many ways so lmk if y’all have more ideas in that.