The Uttershroom
"Climb the fungal-fibre ladders to its summit. Shaggy, suspicious villagers scratch a living here, amidst endless clouds of spores and scurrying mobs of plant-animal hybrids. None of them ever leave. "Monsters," one explains darkly. "Zee full of monsters."
A giant mushroom in the Unterzee, specifically in the region of Myceligaea. There is little animal life here, though fungi thrive. The few people who live here are addicted to prisoner's honey and are too scared of zee-monsters to leave. These inhabitants are related to the people of the Chelonate, who are famed monster hunters; these poor souls didn't want to hunt like their brethren, so left to live here.
The Uttershroom is very likely the original home of the blemmigans. Whether it is or not, they do live here.
