Monster Room

Dungeons were introduced in the "Secret Friday Update 2" to Infdev (and happened to be the last update before Alpha mode was released). They are small rooms made of Cobblestone and Moss Stone in the size of 5*5, 5*7 or 7*7 interior blocks. You can also find rare joined dungeons with 2 spawners and many chests. It is usually not very difficult to spot these, as they have walls made of Cobblestone, which is easy to spot amongst sand/gravel/smooth stone. This is the only place that Moss Stone can be found in a natural map. Caverns are sometimes connected to Dungeons, which leave an opening into the dungeon. Dungeons contain a Mob Spawner and usually one or two Chests that are either large or small. It is possible, although unlikely, to have dungeons with no chests. Dungeons closer to the surface usually have fewer chests. The Mob Spawner acts as a guard against the loot, spawning endless mobs until the player can steal the loot quickly, break the Spawner, or light up the room enough to stop new mobs being spawned. The Mob Spawner is always in the center of the dungeon room with chests located around the walls of the room (large chests can connect with either the short or long side against the wall). The chest contains treasure; common items found are Saddles, Iron Ingots, Buckets, Bread, String and Records. It is possible to find a Dungeon almost anywhere in the ground, but rarely does a Dungeon spawn high in the stone layer of a map. There are reports of dungeons being generated in unusual locations. These are covered by the screenshots below.

Dungeons can be sought out by listening for the sounds of skeletons, zombies, or spiders behind cave walls. As a rule of thumb, hearing the noises of four mobs of the same type usually points to the presence of a dungeon. They may also quickly be located using an up-to-date infinite map viewer. To locate them, set the viewer to show only moss stone (data value 48) or locate cobblestone (data value 4) that you did not place. Play the save file and head out to one such place and dig down until the Dungeon is found. The viewer must be up-to-date, or it may crash upon trying to load blocks that it doesn't recognize.

Dungeon Game Mode
It is possible that adventure mode will be based upon dungeons.
 * See the Adventure Mode Article

Trivia

 * Because deserts are made of sand and dungeons are not generated with a roof, dungeons close to the surface can cause tell-tale rectangular pits. These pits can also be identified by cobblestone or stone exposed by the fallen sand and can be easily looted during the daytime once excavated.