Few encrypted texts are as mysterious – or as tantalizing – as the Voynich manuscript, a book dating to either 15th- or 16th-century Italy and written in a language no one understands, about a subject that no one can figure out, and involving illustrations of plants that don’t exist. Plus it’s got Zodiac symbols, astrological charts, illustrations of medicinal herbs, and drawings of naked women bathing while hooked up to tubes. The manuscript’s 246 calfskin pages were perhaps meant for alchemy or medieval medicine, but no one knows for sure.

What we do know is that it’s written in a language distinct from any European language, and follows a pattern unique to its own. The alphabet ranges from 19 to 28 letters, with an average word length consistent with Greek- or Latin-derived languages, but is missing two-letter words while repeating words at a much higher rate than other European languages. All told, the book has 170,000 characters in it, written from left to right, and there are no punctuation marks.

William Friedman, one of the 20th century’s greatest cryptographers, couldn’t figure it out and suspected Voynich was a constructed, artificial language. (With no Rosetta Stone to help translate.) German computer scientist Klaus Schmeh suspected a hoax, and also suggested the manuscript’s original language could have been encoded in a much larger set of “meaningless filler text.” But there’s no system for separating out the real text from the junk. Linguist and computer scientist Gordon Rugg also concluded the manuscript was a hoax.