Dungeon

Dungeons are naturally generated underground structures that were added in Update 0.9.0. Dungeons are small rooms with a monster spawner and some chests that contain valuables and rare items. The mobs spawners in a dungeon can spawn a zombie, a skeleton, spiders. The monster Spawners can be disable by providing adequate lighting, though.

Loot
It is possible to find the following lot in a dungeon-

Types
Three types of dungeons may be formed

Trivia

 * Dungeon are very hard to locate, though they can be located by listening for mob sounds underground. Note that it must distinctly come from one mob. Also it could just be an indicator of an unlit and dark area nearby. However, dungeons are the easiest of the three underground structures to find.
 * Sometimes, dungeons may not contain chests at all.
 * Before 0.9.4 Alpha, there was a bug that caused the monster spawner to spawn hostile mobs even on peaceful.
 * Dungeon chests may contain saddles. This isn't a glitch, but the saddles still do not function until a future update.
 * Rare items inside dungeons are usually saddles, armor, and valuable ores.
 * Very rarely, one dungeon may intersect the other, forming a double dungeon. Even more rarely, three dungeons will intersect. The chance of such is lower than 5%.
 * In the swamps you could find more easily the dungeons.