What is the dungeon in Minecraft?

Answer 1

A dungeon in vanilla minecraft is a naturally generated rectangular underground room made of cobblestone walls, and containing a monster spawner and up to two chests. Dungeons are sought after by many players, as they can contain rare items such as saddles, contain mossy cobblestone in the floor which cannot be found anywhere else, and the spawner can be used to farm experience or the monsters' drops.

Dungeons are sometimes found connected to large caverns, and are identifiable by looking for cobblestone that you did not place, as dungeons are the only way cobblestone can be generated aside from flowing lava and water meeting. Once you've entered a dungeon, the spawner can be disabled by placing a torch on each side of it, allowing you to loot the chest/s and the mossy cobblestone safely. You'll need to take the torches off if you plan to farm anything from the spawner though.