Monster Room

Dungeons, also referred to as monster rooms in their ID, are structure-like terrain features that appear in the Overworld that contain spawners.

Generation
$$, dungeons generate naturally in the Overworld at any altitude (provided it is next to a cave) regardless of whether "Generate structures" world creation option has been toggled off for said world. For each chunk, 8 attempts are made to generate a dungeon; it is possible, though unlikely, for multiple attempts to succeed. $$, dungeons can generate as underground structures above sea level.

For each attempt, a location and size (an open area with a width and length of 7, 9, or 11 and a height of 6) is chosen. The attempt succeeds if the following conditions are met:
 * The floor area (including under the walls) of the potential dungeon must be entirely solid.
 * The ceiling area (including over the walls) of the potential dungeon must be entirely solid. The ceiling blocks may be gravity-affected such as gravel or sand, which fall if disturbed by the player.
 * The walls of the potential dungeon must have 1–5 openings (2-high air blocks) at floor level.

If the location passes, air and cobblestone are placed, then 3 attempts are made to generate each of two chests. To generate a chest, the chosen block must be empty and must have a solid block on exactly one of its four sides. The mob spawner is placed at the center of the dungeon.

Cobblestone has a $3/4$ chance to be mossy when it is on the floor of the dungeon.

Structure
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. Dungeons generate with either a zombie (50% chance), skeleton (25% chance), or spider (25% chance) spawner. The spawner is always in the center of the dungeon room, with chests located around the walls of the room (large chests can connect with the short side against the wall). Each block of the floor has a 25% chance of being cobblestone and a 75% chance of being mossy cobblestone.

Because the horizontal range for spawning mobs is 4 blocks from the spawner, it is possible for mobs to be spawned outside the walls of a 5×5 dungeon that is surrounded by air outside the walls.

Trivia

 * Dungeons are the earliest added generated structures that still exist in the game today.
 * A contradiction exists in Java Edition that even though the "Generate Structures" world creation option listed "Villages, dungeons etc." as two example structures that are supposed to generate only when the option is turned "ON", dungeons generate even if the option is set to "OFF".
 * Dungeons are always connected to a cavern or other structure, although it's possible for the dungeon itself to overwrite most of a tiny cave.
 * It is possible, though rare, to find connected dungeons with multiple spawners and extra chests.
 * It is extremely rare for a dungeon to generate at bedrock level. When this occurs, the bedrock cuts through the dungeon and can prevent the chests or the spawner from generating. Because the air below Y=11 replaces itself with lava, this cannot happen in caves. However, it can happen in mineshafts.
 * Dungeons are the only structure with chest loot that cannot be located using the command.
 * Unlike most other generated structures that generate in positions determined by world seed, dungeons are generated in a very different way by performing checks per chunk, as mentioned above, similar to how trees are generated. As a result, they can, in a way, be considered "underground trees".