This relief appears on an 18th-century monument on the grounds of Shugborough Hall, a country estate in Staffordshire, England. The shepherds are pointing to this inscription:

    O•U•O•S•V•A•V•V
D•                   M•

What does it mean? No one knows. If it’s a ciphertext, no one’s been able to solve it, and that includes the codebreakers at Bletchley Park, who cracked the German Enigma code in World War II.

If you can decipher it, there may be a reward in it for you: Some say the message contains a clue to the location of the Holy Grail.