Voynich Manuscript (pictured at left) is currently one of the bigger linguistic mysteries out there . It is a set of some 240 hand-illuminated pages, bound in codex form, making what appears to be a reference work on such topics as herbalism, biology, and astronomy. Many of the illustrations are of plants and flowers that do not actually exist or cannot be precisely identified. Most puzzling is the text, which is written in an unknown and undecipherable script that bears no relation to any known language or script. You can see high-quality scans of the book here, courtesy of the Yale Library.
It is believed that the manuscript is a pharmacopoiea, as it bears some similarities to other such works. However, much of it is puzzling, and incomprehensible. Some scholars have proposed the manuscript to be a fake, one of a number of herbals made in the Middle Ages by alchemists and charlatans to impress simple people with the possessor's supposed knowledge. The text is gibberish, mere squiggles on a page, meant to look like writing and yet containing no message. That's one proposal.
Yet, the script looks intentional. The same letters are repeated, and even specific ligatures are discernible. The letters are repeated in such a way that shows consistency, as though the author were writing in an actual script, and not merely scribbling.
There are all kinds of hypothesis about how and why the manuscript was authored. The most plausible is probably that the text is an invented script meant to write an East Asian tonal language. Other theories are that it is a secret script or language invented by the author to hide his writing, or that the script is a code, containing information in some secondary feature of the words.
Those are the best theories.
But I want to propose a crazy theory, and a way to test it.