A hard to navigate, low visibility deep cave system with kelp everywhere, or whatever the Proteus equivalent of kelp would be. It would have murky water, and some sort of predator (Not a leviathan, maybe another mango.) that hunts with electroreception and smell, and also sound. (like sharks do in real life.) This would be a really good use of the cavitation muffler upgrade, as this predator would be completely blind. The biome could also be the location of a sonar upgrade, as sonar would make navigating the biome much easier (Like the upgrade from subnautica 1) See post #8890