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 Mossy Cobblestone. This is the only place that Mossy Cobblestone can be found in a natural map, so it might be wise to mine it. Caverns are sometimes connected to Dungeons, which leave an opening into the dungeon. Dungeons contain a Mob Spawner and one or two Chests, although some players have reported finding Dungeons without any Chests, dungeons closer to the surface have less of a chance of having chests in them. 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. The chest contains treasure- Saddles, Iron ingots, Buckets, Bread and String are a few things often found. 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. However, a player has reported finding a Dungeon right under 3 blocks of their spawn location. Other users have taken pictures of Dungeons set into a hillside and buried by sand in the ocean.

Dungeons can quickly be located using an up-to-date infinite map viewer. To locate them, set the viewer to show only mossy cobblestone (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
See the Adventure Mode Article

A video of a Dungeon being found.