Strongholds are abandoned underground structures found in Minecraft, but they are relatively rare and actually finding one can be difficult. This is a seed you can use to find a stronghold right away, it spawns you pretty much right outside a very large minecraft stronghold.
Here’s a seed guaranteed to spawn you just a little ways away from a stronghold:
Here are the co-ordinates for this massive mineceraft stronghold, the glass tower is due to this screenshot having been taken in the 1.9 pre-release and probably won’t be there by the time you get to it.
The stronghold is an undersea stronghold and very extensive indeed, including an abandoned library. Here’s a couple screenshots to whet your exploratory appetite: