Yale University's Beinecke Rare Book Library is in possession of many fascinating historic texts, but perhaps its most perplexing item is a 600 year old, one-of-a-kind book known as the Voynich Manuscript. The textual content of this mysterious illustrated book is unknown because it is written in an unknown alphabet and unintelligible language, and despite the efforts of expert cryptographers over many decades, not a single word has been deciphered.
Curiosity of the book's content is fed by the bizarre, elaborate illustrations that appear throughout its pages. The manuscript appears to be made up of several sections, each containing distinctly different illustrations which shed little light on its contents. These illustrations include plants, some of them unidentifiable; astronomical/astrological charts; and groups of nude women bathing in tubs and pools shaped like human organs.
The book is named for Wilfrid M. Voynich, a Russian-American book dealer who acquired the manuscript in 1912. The book contains about 240 parchment pages, but appears to be missing several pages as evidenced by gaps in the page numbering. The actual origin and date of the book are vigorously debated, though most agree that it was written in central Europe in the late 1400s or during the 1500s. Several plant drawings inside the book have been identified as specimens from North America, so it is presumed that the book must not predate Columbus's voyage to the New World in 1492.
Despite the fact that statistical analysis of its text reveals character patterns similar to natural languages, more than a few people are convinced that the manuscript originated as an elaborate hoax... nothing more than arbitrary symbols arranged in a meaningless order. But because that contention is inherently unprovable, and because the manuscript's patterns seem to reflect real information, many experts and amateurs continue in efforts to decipher this holy grail of historical cryptology. Several individuals have separately claimed successful decoding of the text, but each such decoding relies on broad, unfounded guesses, rendering the results useless.
Update, 19 February 2014: University of Bedfordshire professor Stephen Bax may have taken the first steps toward decoding the manuscript. Time will tell.