Coded mysteries come in many forms and sizes, from the four-letter tetragrammaton to the 240-page Voynich Manuscript. One of the most enduring is comprised of ten letters inscribed on an architectural folly on the grounds of an English country estate in Staffordshire. Between a divided and vertically offset “D” and “M” falls the cryptic sequence “OUOSVAVV.” Visitors to Shugborough Hall, a Neo-Classical mansion built in 1693 by William Anson, will find the letters on the Shepherd’s Monument, so named for its adaptation, in marble relief, of Nicolas Poussin’s famous painting, The Shepherds of Arcadia. (This creates a kind of riddle inside a riddle, as the painting depicts shepherds crouched around a tomb, the epitaph on which — Et in Arcadia ego — has been been the subject of its own lively debate.) People have been puzzling over the Shugborough Inscription ever since the monument’s completion, sometime between 1748 and 1763, with interpretations ranging from a coded dedication to George Anson’s deceased wife to the simple initials of prominent Shugsborough citizens. Worldwide interest in the mystery skyrocketed in 1982, however, after Michael Baigent, Richard Leigh, and Henry Lincoln–authors of the controversial book The Holy Blood and The Holy Grail (whose theories were later fictionalized by Dan Brown in The Da Vinci Code)–asserted the inscription held clues to the current whereabouts of the Holy Grail.