You're thinking of the Cavern of Remembrance in Radiant Garden. Proof of Existence is the area in TWTNW with all the gravestones right before where you fight Xigbar.[DOUBLEPOST=1374073359][/DOUBLEPOST] I just thought that was Castle Oblivion's Keyhole, which would have changed places after Aqua created it from the Land of Departure. Sora probably just locked it because it was another Keyhole that hadn't yet been locked. Of course, this is based on the original Chain of Memories, which I remember much better than Re:COM.