I remember this as my favorite map about 3 years ago... since have lost it.
Its a large floating cube with a cave entrance coming out of the top on one corner. This cave held various mob spawners & supplied loads of monsters.
Next to the main cube there was a second smaller island, but it was just as deep. On this one you would travel down a very deep hole of a cave and at the bottom there was a chest and more mob spawners. That's it, just the two islands floating out in the middle of nowhere.
If you have any thought as to what it might be, please let me know. I might even be a little fuzzy on the details so if it's anything close feel free to send it over.