I have this romantic idea about the lost writings of Epicurus that perhaps somewhere at the bottom of the Aegean Sea there still lies a ship sunken in Ancient Greek times with copies of the philosopher's writings stored in some waterproof amphorae (or something like that). Maybe one day they will resurface as was the case with the Antikythera wreck and mechanism, or maybe they won't and the ship and its remaining contents will just dissolve into enlarging entropy. Personally, I would certainly welcome their discovery most dearly ...

Now my question is along similar lines but more concrete in terms of known history: What were important instances of historic documents (or other artifacts) that were thought lost (e.g. with the library of Alexandria) or that were inaccessible at the time (e.g. in Soviet Russian archives) and that led to significant new historic understanding when new developments occurred and copies became available (perhaps surprisingly)?