Some FF fans who tweaked some FF 13 files have discovered an unused dungeon from Final Fantasy XIII called The Seventh Ark. It seems that this area was intended to include quests and a free battle area, but Square-Enix decided to cancel it “because of the great differences between the PlayStation 3 and Xbox 360 network systems”.
As we can see in the footage below, The Seventh Ark dungeon includes several cut scenes and even a boss fight with Nemesis, an enemy similar to to the fal’Cie Anima, so it has been speculated that S-E could have planned to release this dungeon as DLC. The real fact is that, finally, the company decided to eventually use the interior of The Seventh Ark in Final Fantasy XIII-2 for the battle against the demi-fal’Cie Adam.
Here you can watch a playthrough video of this unseen dungeon: