Shepherds Monument, Staffordshire. Built in 1748, it is not the oldest but is perhaps the most mysterious in the series to date. Below a mirror image of Nicolas Poussin's painting of the Shepherds of Arcadi, the letters O U O S V A V V, between the letters D M are carved on the 18th-century monument. It has never been satisfactorily explained, and remains to be one of the world's top uncracked cyphertexts. Reference was made to the monument in the pseudohistorical book 'The Holy Blood and the Holy Grail', which inspired Dan Brown's infamous Da Vinci Code. In 2004 a project was founded where a group of former members of the top secret wartime Bletchley Park code breakers attempted to decode the connection between the monument and the holy grail. Despite the fact that organisers had their own favoured theories, no conclusive answer ever emerged, and the cyphertext remains a total mystery.