system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is it that every year there's a new WWII documentary with "never before seen" footage? Hasn't all the footage already been seen /released?

For as long as I remember WWII documentaries usually have the same footage and its been that way until maybe 7 years ago where channels like National Geographic & History started showing documentaries with "never before seen" footage.

For instance the other day I saw two documentaries about Auschwitz and Bergen-Belsen that showed pretty graphic footage and it was being "advertised" as never seen. They actually talked about Nazi movies still being banned in Germany until this date.

So... to rephrase the question, where is all this footage coming from? Is it that some countries never released it? They just found out it existed? Its amazing that none of it has leaked to the internet (at least that I know). I would expect that almost 100 years later every photo and video have already been seen. Seems like I'm wrong!

Edit1: Spelling is hard.
gpt
In addition to what other posters have said, a lot of footage can wind up in archives and other storage locations where they aren't digitised, or are even mis-labeled or mis-placed, so it's very hard to search. Someone has to physically go through a warehouse containing god knows how many thousands of film reels, and examine them. Historians doing research can come across such recordings, and realise that they contain new footage or photographs that had not been available before. On the other hand, there could be situations where they knew film existed, but it was damaged, and only recently has it been fully restored.