Didn’t they lose a large part of Einstein’s brain (or not realize it was stolen, so everybody but one person forgot)?
They also lost/forgot about a lot of the Apollo 11 tapes; not sure if they were audio or video.
Changing OP here, however.
City of Ur? It was truly lost and was once the biggest city on the planet.
I don’t have the details handy, but in Guns Germs and Steel Jard Diamond discusses a culture that lost, at sea, a massive stone wheel that served as the largest denomination of their currency - something like the ‘trillion dollar coin’ people are proposing now.