The Voynich Manuscript

The Voynich manuscript is an illustrated codex hand-written in an unknown writing system. The vellum on which it is written has been carbon-dated to the early 15th century (1404–1438)

[…]
The Voynich manuscript has been studied by many professional and amateur cryptographers, including American and British codebreakers from both World War I and World War II.[19] No one has yet succeeded in deciphering the text, and it has become a famous case in the history of cryptography. The mystery of the meaning and origin of the manuscript has excited the popular imagination, making the manuscript the subject of novels and speculation. None of the many hypotheses proposed over the last hundred years has yet been independently verified."

Source : Wikipedia

Researcher Nicholas Gibbs recounts his attempts at deciphering in The Times Literary Supplement, Set. 2017

Update Apr. 2019 : A paper in the journal Romance Studies claims to have solved the encryption. But these claims are strongly refuted by various academics in the field.

Update May 2020 : Although some scholars have dismissed the manuscript as a hoax, a 2020 study which computational methods to analyse 'word' frequency suggest that it's not:

In this paper, we showed more support for the claim that the VM is written in a natural language and therefore is not a hoax. Although several scholars have found statistical evidence pointing in the same direction, more evidence is needed, particularly to establish whether there is a known language family to which VM can plausibly be assigned.

Source : Language Resources and Evaluation Conference (LREC 2020)