Monster Room

Dungeons were introduced in the "Seecret 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. While you may not find any close by in one map, in the next you could find three of them close to your spawn point. Although it would be dangerous to wander into a dungeon unprepared, you shouldn't have to worry. They are easy to find, as they have walls made of Cobblestone, which is easy to spot amongst sand/gravel/smooth stone. You will also hear lots of sounds from the same type of mob. Dungeons are the only places that Moss Stone can be found in a natural map. Caverns are sometimes connected to Dungeons, which leave an opening into the dungeon, releasing a flow of mobs, causing a very unexpected danger during a mining excavation. Dungeons contain a Mob Spawner, which can be used to make a mob spawner trap, and usually one or two Chests that are either large or small. It is possible, although unlikely, to have dungeons with no chests, but there was one inside 0.0.0.0:25565 without chests. Darkus Relling(aka DrRelling) already searched the room and removed the spawner. 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 Bread, Bucket, Iron Ingots, Records, Saddles, Strings, Redstone Dust, Sulphur and Wheat. More rarely, one finds Apples and Golden Apples. 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), mob spawner (data value 52) or 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

Tips

 * An easy way to find dungeons is to hit F, the fog key, and while the screen is blue with clouds and a sun/moon, look for flames from a mob spawner.
 * One way to locate dungeons is through listening, either for a large amount of noise from one kind of mob, or for ambience. Neither of these things is an exclusive trait of a dungeon, but they do mean that there is an unlit and unexplored area nearby, which may well be a dungeon, particularly in the case of frequent and similar mob noises.
 * To defeat a dungeon, besides the obvious path of fighting the monsters and lighting up the area, one can instead tunnel underneath it and destroy the spawner from below, or tunnel into the walls and remove the chest contents from behind.

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.
 * Sometimes dungeons can be found on the coast on beaches. It is similar to desert dungeons since the sand will cave in. Furthermore, water can flood into the dungeon as well.