Well I was exploring minecraft one day inside of a stronghold, and suddenly, at the VERY bottom of the stronghold, I found this. It seemed to be some sort of superdungeon... There was a dungeon with a mob spawner but the opening was only 1 block wide/tall, though I don't know what it spawns. There was also a ladder going up into a room with a wooden floor with a double check and a hole leading down to the cobblestone well. (See Picture Above)