For three years, Ringelblum and his researchers amassed thousands of documents, including eyewitness testimonies, diaries, resistance materials, poems, paintings, sketches, maps, recipes, surveys, and even jokes:
Hitler comes to the other world. Sees Jesus in Paradise. "Hey, what's a Jew doing without an armband?"
"Let him be," answers St Peter, "He's the boss's son."
If the Nazis discovered the work of Oneg Shabbat, shit would hit the fan, and bullets would hit the bodies, so secrecy was paramount to the group. Collecting and compiling documents was done on carbon paper -- so if one copy was destroyed or damaged, there was another copy out there. The collection was also split up across several properties inside the Ghetto, for extra security.
In 1942, the Nazis began liquidating the Ghetto (precisely what that sounds like), so Ringelblum, knowing that was time was short, had the cache divided into three smaller caches. Each was secured and waterproofed in metal boxes and milk cans, and hidden in the walls and cellars of three buildings inside the Ghetto. In 1943, Ringelblum and his family were smuggled out of the Ghetto. They remained hidden in Warsaw until 1944 when they were discovered and executed.
As for the archive, one cache has yet to be located, but two of the others were recovered in 1946 and 1950, serving as an unparalleled record of life in the Ghetto.