Minecraft wiki entry for Cave, associated text: The deep dark is an overworld biome that generates deep underground (below Y = 0), usually in mountainous areas with high continentalness. The floor in this biome often has 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 the sculk shriekers a certain amount of times, the Warden may appear, giving the player the darkness effect.