Underground Deep Sea/Abyss Biome

With the new underground aquifers found in Caves & Cliffs, a new cave biome could be one that exclusively generates deep underground (perhaps only under y=0 or y=15) and only generates as aquifers: the Abyss biome. This biome would generate with (perhaps) glow squids, maybe even a deep-sea Anglerfish-type mov, and the sky/water fog color would be pitch black. New structures and plants could generate here, with tube worms and deep-sea vents. This would be a really fascinating biome to encounter. Perhaps it could only generate under Ocean and Deep Ocean biomes?