Ceridwen

In 1912 in northern Italy, book dealer Wilifred Voynich bought an illuminated codex in an unknown writing system. Containing about 240 pages (though some are missing), the manuscript is written on vellum, and has been carbon-dated to the early 15th C. The manuscript has long been assumed to be a cipher - though that wasn't exactly proven until analysis discovered a semantic pattern in the text; it could have been gibberish - and both amateur and professional codebreakers have taken cracks at it over the years. Nada. From this void of understanding, all manner of crackpot theories have emerged, with an emphasis on the alien/Atlantean memeplex of doom. Having a 15th Century unbroken cipher is like the god of the gaps of the secret history. 

But now, there's been a pretty significant breakthrough with the manuscript. A botanist and an information technologist (I think this is a fancy term for librarian) compared the botanical illustrations with plant distributions of the time of the manuscript's first recorded appearance, which is the late 1500s. (Why this doesn't jibe with the carbon dating, who knows. Maybe the vellum was produced earlier or something; I'm no archivist.) The researchers eventually identified 37 plants and six animals in the codex from the New World, specifically post-Conquest Nueva España (New Spain). (Which is totally bananas on several levels; badumptss.) In addition to the New World plants, there appear to be similarities with a Mexican codex written at the roughly the same time, and the captions on many of the plants are in Nahuatl. 

Once you place the manuscript on the right continent (or planet, even), it's just a matter of time to break the whole thing. Like the Navajo code talkers, the codex wasn't exactly in code, it was in a (now) obscure language. Given that the Spanish put so much of Central and South America's cultural history to the match, the Voynich manuscript could end up being this profoundly important window into lost history. Jeepers, that's just the coolest.