What It Was In Its Past Life?
Cyrex686 had at long last solved the mystery of what had been housed in this enigmatic room in a previous incarnation; however, what he discovered next completely blew his mind.
The fact of the matter was that when cisterns are no longer required in a building, it is extremely typical for it to be very common for them to simply be closed off from the rest of the structure, as opposed to the time-consuming and costly process of removing them.
This suggests that Cyrex686 had actually taken much greater risks than he had ever realized; for example, he could have caused a massive leak of water or sewage, which would have caused it to flood directly into his home. He also ran the risk of causing an explosion, which would have resulted in his death.