I have come to the Beinecke Rare Book and Manuscript Library, at Yale University, to solve a mystery that makes The Da Vinci Code seem tame - a book no-one can read, in a language that doesn't exist, illustrated with plants and creatures that have never been seen on Earth.

It is known as The Voynich Manuscript, after the second-hand book dealer, Wilfrid Voynich, who claimed to have discovered it in Italy, in 1912. Since then, it has obsessed countless experts and generated numerous theories, both scientific and crackpot. "My favourite is that it is the illustrated diary of a teenage space alien who left it behind on earth," jokes the Beinecke's curator, Ray Clemens.

Bound in a limp vellum cover the colour of old ivory, it contains 240 richly illustrated pages. The illustrations look like something Timothy Leary might have seen on LSD. Strange plants, astrological symbols, jellyfish-like creatures and what looks like a lobster. In one image, a group of naked ladies with alabaster skin shoot down what looks like a water slide. The text, written in brown, iron gall ink, reminds me of Tolkien's Elvish.