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 between 0 and 2 Chests. Dungeons are the only places that Moss Stone can be found in a natural map. Anyone exploring a cavern should be wary of encountering a dungeon, as they are almost always connected to a good-sized one, and are very dangerous. Dungeons exist on peaceful mode, as they are generated with the map, however the Monster Spawner will not spawn any monsters.

Properties
Dungeons are generated as a 5x5, 5x7, or 7x7 floor space surrounded by Cobblestone or Moss Stone. Dungeons contain a Monster 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 uncommon, to have dungeons with no chests. It is also possible to find rare joined dungeons with 2 spawners and 0-4 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 light up the room enough to stop new mobs from being spawned, or simply 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 the short 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 gallery below.

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

In a future update, Notch may add boss monsters into dungeons

Chest contents
Each dungeon chest block contains up to 8 item stacks, 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. This tends to work better on slower computers.
 * Another 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 are 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.
 * 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.
 * To defeat a dungeon, aside from the obvious method of fighting the monsters and lighting up the area, one can instead tunnel underneath it and destroy the spawner from below. or it is also possible to flood the dungeon with lava, kill all of the mobs, then place torches to disable the spawner just as the last of the lava disappears.
 * Digging around the sides of the dungeon will allow you to bypass the spawner and loot the chests directly without any risk of health loss.
 * A fast way to collect loot from the chest is to destroy the chest rather than hand dropping all the items. This way you collect all loot AND the chest.

Trivia

 * Dungeons are always connected to a cavern, and because the cavern can be as small as a single block of air, the dungeon could completely overwrite it.
 * 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. There is also a chance that the dungeon might be turned upside down. This is most likely a generation error. You may also find partially-generated dungeons near chunks that were re-generated.
 * It is possible for a dungeon to be generated at bedrock level and deleting the bedrock, as such digging through the moss stone can lead into the Void.
 * It is possible for a Dungeon to spawn over a lava pit. It is also possible that the dungeon does not have a cobblestone floor, or only a partial floor, resulting in a floating spawner.
 * Rarely, on peaceful, a monster spawner may spawn a monster that plays its sound file before it can despawn.
 * Rarely, a monster spawner can be spotted from the surface. Islands of gravel or sand will have an obvious ditch in them which can indicate a monster spawner which the sand or gravel has caved in on.
 * It is rarely possible for a player to find a dungeon with two spawners and up to 4 chests, this occurs when a 5x5 dungeon spawns within a 7x7 dungeon, since the dungeon will remove any blocks from the walls or floor that open into a cave, the walls of the smaller dungeon will be removed, resulting in a 7x7 floor space with one spawner in the middle, and a second spawner adjacent or diagonally adjacent to the first.
 * Sometimes, double chests in dungeons are spawned perpendicular to the wall as shown below (W = Wall, F = Floor, C = Chest, M = Mob Spawner):

WWWWWWW WFFFFFW WCCFFFW WFFMFFW WFFFFFW WFFFFFW WWWWWWW
 * Sometimes, dungeons spawn at the lava level. This causes parts of the floor to be open directly to the lava.
 * The Moss Stone pattern on the dungeon floor seems to have a patttern, most noticeable with a 5x5 dungeon, where a 2x2 cube of moss stone lays in one corner. Unknown if this happens with 5x7 dungeons and, of course the larger 7x7.
 * Although extremely rare, three dungeons may spawn together or very close to each other, resulting in a massive amount of mobs and up to 6 chests. Due to the sheer amount of mobs generated, these dungeons may be very difficult to conquer.

Gallery
File:2011-07-17_16.07.58.,png A pig dungueon on multiplayer