The Voynich Manuscript

The Verge is probably correct to call this 600 year-old manuscript as "the most mysterious book in the world".

There is no default, accepted theory to explain the manuscript's provenance, and any theory that gains traction is usually disproven or disregarded by the huge community of amateur and professional Voynich scholars. Ten years ago news reports appeared suggesting the document was a hoax, written 100 years after its carbon-dated vellum suggests. It might've come from Mexico. Or it could've been a philosophical experiment, or a work of art or, according to theoretical physicist Andreas Schinner, put together by an "an autistic monk, who subconsciously followed a strange mathematical algorithm in his head." Or aliens wrote it.