The Voynich Manuscript
The Voynich Manuscript is a 15th-century Italian text that looks pretty unassuming. The handwritten codex seems to be full of all kinds of medieval knowledge on various subjects, possibly even holding ancient secrets. There's only one problem: it's scripted in some indecipherable language. The greatest cryptographers and linguistic masterminds in history have hacked away at it for centuries, but nobody has ever been able to crack it. What lies inside of the impenetrable book?
Fortunately, it comes with pictures. Unfortunately, they make literally no sense. It seems to be some sort of guide, containing illustrations of nymphs, astrological charts, and herbology records. Some think it's a women's health manual, while others are convinced it's an almanac. It's not very clear what language it's written in either. Lots of archaeologists (and AI programs) think that it's written in some sort of "vulgar Latin dialect" in a form of weird Roman shorthand.
Although the true secrets of the Voynich Manuscript have never been uncovered, perhaps someday in the future they will be. Until then, we only have our imaginations to believe.