Complex sentence: The plan worked so well that to this day it is unknown where exactly the treasure was hidden in that time--it is only certain that it was packed in cardboard and shabby suitcases and taken to some place in the diocese of Hildesheim.
Split into simpler sentences: The plan worked so well that to this day it is unknown where exactly the treasure was hidden at that time. The only certainty is that it was packed in shabby cardboard suitcases and taken to some place in the Diocese of Hildesheim.