The book nobody has been able to read in six centuries

In Yale University's Beinecke Library there is a manuscript catalogued as MS 408. It has some 240 pages of parchment, is written in a fluid, confident hand, and is illustrated with plants, astronomical diagrams and small figures bathing in green pools. There is just one problem: nobody knows what it says.

The alphabet matches none we know. The plants correspond to no real species. Carbon dating places the parchment in the early fifteenth century, so it is no modern invention: someone wrote it about six hundred years ago, fluently, like someone at ease in their own language.

The best have tried. William Friedman, the cryptographer who broke Japanese codes in the Second World War, devoted decades to it. Computer teams, linguists, historians. There are theories to suit every taste: a lost language, a cipher, a very expensive hoax. None has won.

It is called the Voynich manuscript, after the bookseller who bought it in 1912. Anyone can see it: it has been digitised in full. We remain exactly as illiterate before it as we were a century ago.