Minecraft wiki entry for Cave, associated text: Dungeons, also referred to as monster rooms in their ID, are structure-like terrain features that appear in the underground that contain spawners. Dungeons are small rooms made of cobblestone and mossy cobblestone and contain a mob spawner and up to 2 chests. Finding a dungeon without a chest is unlikely but possible. Additionally, there is a rare chance of a dungeon being found without a spawner. Occasionally, a dungeon generates with its chests in such a way as to create a double chest.