Monster Room

Dungeons were introduced in the "Seecret Friday Update 2" to Infdev. They are small rooms made of Cobblestone and Moss Stone that contain a hostile Mob Spawner and a few Chests. 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.

Properties
Dungeons are generated as a 5x5, 5x7, or 7x7 floor space surrounded by Cobblestone or Moss Stone. Dungeons contain a Mob Spawner, which can be used to make a mob spawner trap, and up to two Chest blocks (which may be adjacent to form a large chest). It is possible, although unlikely, to have dungeons with no chests. You can also find rare joined dungeons with 2 spawners and many 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, light up the room enough to stop new mobs from being spawned, or just break the spawner itself. 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 chests contain randomly generated treasure that are placed in random slots. It is possible to find a Dungeon almost anywhere in the ground, but they rarely spawn high in the stone layer of a map. There are reports of dungeons being generated in unusual locations, as shown in the screenshots below.

There is a 50% chance the spawner will be a zombie spawner, 25% chance of skeletons, and 25% chance of spiders.

Chest contents
Each dungeon chest block contains up to 8 items, with the following distribution:

This information was obtained from the Minecraft code. The generation algorithm places one of these items in a random slot in the chest eight times, possibly overwriting items already placed.

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. This only works when you are 16 blocks or less from the 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.
 * An incredibly easy way of locating Dungeons was included in Update 1.3_01. Simply pressing F3 places data values above all Mobs, all of which can be seen through blocks (similar to nameplates in Survival Multiplayer). Simply pressing F3 and looking for a large concentration of Mobs underground will likely result in the discovery of a dungeon.
 * If all else fails, one can use a map viewer to search for Moss Stone blocks. 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.
 * A faster way to collect loot from the chest is to destroy the chest rather than hand dropping all the items. That way you collect all loot AND the chest.

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.
 * It is possible for the player to spawn in a dungeon near the surface.
 * Rarely, a mushroom will appear in a dungeon on mossy cobblestone.
 * Very rarely, there will not be a mob spawner in the dungeon. This is most likely a generation error. You may also find partially-generated dungeons near chunks that were re-generated.