The Shugborough inscription is a sequence of letters - O U O S V A V V, between the letters D M - carved on the 18th-century Shepherd's Monument in the grounds of Shugborough Hall in Staffordshire, England, below a mirror image of Nicolas Poussin's painting, the Shepherds of Arcadia. It has never been satisfactorily explained, and has been called one of the world's top uncracked ciphertexts. The inscription became widely known after being mentioned in the 1982 book The Holy Blood and the Holy Grail by Michael Baigent, Richard Leigh, and Henry Lincoln.
The Wikipedia page is  and gives a list of various theories. There is also various other coverage on the web.