Alright, just to clear things up.
1. Mob spawners don't spawn random mobs, just the ones inside of them.
2. Mobs spawn in darkness, I think it's a light level of 5 or less...
(torches give off a light of 7 or so, meaning that the block they're on has a light level of 7, the ones next to it have a level of 6, and so on.)
3. When you find a dungeon, which is a cobblestone square underground, partially made of mossy cobblestone and usually contains 1-3 chests and 1-3 mob spawners, it is a square. Mob spawners can only spawn mobs about 7-8 block from them and the whole square is about 7x8 blocks. This means that ether he mined right through one and failed to realize it, or there was a word generation error.
4. If you do happen to find a mob spawner, just light up the whole dungeon with torches and then look up " Mob spawner EXP farm" and I think you'll be happy you saved it.
-Brett
