Minecraft wiki entry for Defeating_a_dungeon, associated text: Dungeons generate at any altitude provided they are next to a cave, even if "Generate Structures" is turned off. When mining or exploring underground, if you stumble along naturally generated cobblestone, that's a good sign you might have found a dungeon, but water and lava mixing also generates cobblestone, so be careful when investigating. Mossy cobblestone generates a lot in the floor, and there are only five naturally occurring sources of mossy cobblestone: dungeons, jungle temples, pillager outposts, ocean ruins and mossy cobblestone boulders. So if you're not in a jungle biome or aboveground, you've found a dungeon.