Monster Room

A monster room is a structure-like feature that appears in the Overworld that contains a monster spawner that produces either zombies, skeletons, or spiders.

Generation
$$, monster rooms generate naturally in the Overworld in any biome. Each chunk has fourteen attempts to generate monster rooms underground within y-coordinates 0 to 320 or -58 to -1, each altitude range with different uniformity. For the higher altitude Y levels 0 to 320, up to 10 monster rooms can generate, while up to 4 monster rooms can generate in lower altitude Y levels -58 to -1. It is possible, though unlikely, for multiple attempts to succeed. $$, monster rooms can generate as underground structures above sea level.

Monster rooms are a feature instead of a structure, so they generate regardless of whether the "Generate structures" world creation option has been toggled off for said world. They also cannot be searched by the command.

For each attempt, a location (the position for monster spawner) and size (an open area with a width and length of 7 or 9 and a height of 6) is chosen. The attempt succeeds if the following conditions are met:
 * Blocks in the floor area (including under the walls) of the potential monster room must be material solid.
 * Blocks in the ceiling area (including over the walls) of the potential monster room must be material 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 monster room must have 1–5 openings (2-high air blocks) at floor level. Monster rooms are always connected to a cavern or other hollow structure, although it is possible for the monster room itself to overwrite most of a tiny cave.

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 monster spawner is placed at the center of the monster room.

The monster spawner has a 50% chance of producing zombies, a 25% chance of producing skeletons, or a 25% chance of producing spiders. A zombie monster spawner $$ also has the same chance (5%) of producing zombie villagers as in natural generation. Cobblestone has a 75% chance to be mossy when it is on the floor of the monster room.

Construction
Monster rooms are small rooms made of cobblestone and mossy cobblestone and contain a monster spawner and up to 2 chests. Finding a monster room without a chest is unlikely but possible. Additionally, there is a rare chance of a monster room being found without a monster spawner. Occasionally, a monster room generates with its chests in such a way as to create a double chest. Monster rooms generate with either a zombie (50% chance), skeleton (25% chance), or spider (25% chance) monster spawner. The monster spawner is always in the center of the monster room 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 monster spawner, it is possible for mobs to be spawned outside the walls of a 5×5 monster room that is surrounded by air outside the walls.

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Trivia

 * 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", monster rooms generate even if the option is set to "OFF".