Minecraft wiki entry for Deep_Dark, associated text: The deep dark is an Overworld biome that generates deep underground in areas with low erosion. This biome is often covered in large patches of sculk blocks surrounded by sculk veins, which include sculk-related blocks such as sculk shriekers, sculk sensors and sculk catalysts. No mobs spawn in this biome, but if a player triggers a sculk shrieker a certain amount of times, the warden may appear, giving the player the Darkness effect. This is the only biome where ancient cities can generate. This biome is very dimly lit as there's no glow lichen, with the only natural light sources being sculk catalysts and lava. Mineshafts can't generate in this biome, but they can intersect it if generated in another biome.