Today I came across this article on my phone's Flipboard app.
http://www.collective-evolution.com/2013/01/17/6-baffling-discoveries-that-science-cant-explain/
I have since spent the last four hours reading and digging for information on the first subject alone (the Voynich manuscript), which has apparently been the subject of many interesting theories and novels. The book consists entirely of some completely unintelligible language that no one has ever seen or heard of, and is also filled with some very curious illustrations, some of which fold out to reveal some very complex diagrams. Carbon dating places it to be written sometime in the 1400's. Based on the illustrations, it appears to be divided into sections, the first dealing with various plants-- the rest not making much sense.
A PDF of the book can be seen here:
http://awesta.sibirjak.ru/files/Voynich.pdf
Some people have suggested that it is a hoax, fabricated by Voynich himself, being a dealer of old and unique books. Some suggest it is encrypted somehow, or is written in a native "natural" language that is in no way related to any of today's languages.