Linguistics
We can read every letter of this language and still don't know what it says
Etruscan, spoken in ancient Italy before Rome's rise, was written in an alphabet borrowed from the Greeks, and scholars can pronounce its inscriptions with confidence. But because so few bilingual texts survive and it isn't clearly related to any other known language, most of what the words actually mean stays uncertain. It's a mystery of a different shape than the Voynich manuscript: not an unreadable script, but a readable one whose meaning stays out of reach.