Dungeons are rare to some people they contain mossy cobble stone and a mob spawner ( spawns monsters only besides slimes) just keep on digging around don't use dynamite on one it will destroy the chest in side that sometimes contains rare items like saddles.

What a lot of people don't know is that when you are about 15-20 blocks away from a dungeon in any direction, some kind of short creepy noise like a sudden shriek or a drawn out low piano noise. Sometimes after that you can hear the sounds of either the Spiders, Skeletons, or Zombies spawning from the dungeon (Creepers don't spawn in dungeons). If after all that your still not able to find one, your just unluckey

try to hear a creepy sound under the cave but when you found it. DO NOT try to break it.