mycelium should also generate underground

Along with the lush caves it feels like we should either move the mushroom island biome underground or add in a new fungal cavern biome with mycelium as the floor and perhaps some giant glowing mushroom types as well to give it a nice flavor. . . maybe something similar to what we see in the nether with shroomlights.

Allowing Mooshrooms to spawn underground as a passive mob also would go a long way towards letting players build and play entirely below the surface. They would be a nice food source for an area where you probably don't want to allow us to grow crops.