Minecraft wiki entry for Development_versions, associated text: Generates within the Deep Dark biome and includes all of the Deep Dark features within it.
Made of deepslate and its variants, basalt and its variants, and gray wool.
Contains soul sand, soul fire and soul lanterns, as well as candles and skeleton skulls.
Contains chests with unique loot, like the Swift Sneak enchantment.
Has different levels and open spaces.