The Observer has stated that there were several witnesses and accounts of the event, but all those who witnessed the event were "exterminated", and all but one document of the event was destroyed: the journal of a Nazi soldier named Sebastian Kraus. The Observer says that the journal was a "form of memory" of this event and it holds "forbidden knowledge". When asked what this forbidden knowledge is, the Observer simply says that "the Nazis knew more than you think they did".
The journal is currently in Karl Maxwell's possession. Karl possibly found it in an encounter with the Administrator during his service in the OSS, the United States' anti-Nazi intelligence organization. As of the video "SCRINIARII" it is in possession of a entity named Scriniarii.
In "Severance", HABIT says that he was working with the Nazis in some kind of experiment involving the Administrator, the Collective and Sebastian Kraus, the author of Karl's journal. It is not clear what this experiment was nor what the results were, but it is known that the contents of the journal are extremely important; so important that it possesses the ability to ward off the Collective.
In the TRIBETWELVE VETERANS DAY SPECIAL livestream, Future Noah Maxwell mentions the event as being very, very important; describing it as the "whole reason everything is happening". According to him, the event is also why 11/11 is extremely important.