Great Holy Wall

Is a really big, plain wall in Seireitei. Nobody today has an idea why it is called 'Holy' or what is so remarkable about it besides being big. Many agree that the big empty wall looks boring. There were suggestions that some decoration should be added to it, but so far everything has been rejected. The authorities are against any decorations in public places, insisting that everything has to look as bland and boring as possible. Nobody knows why either.
but rumor has it that there have been cults seen going in and out of a trap door in the side.