According to legend, when the pirate Olivier Levasseur was hanged in 1730, he flung a necklace into the crowd, crying, “Find my treasure, the one who may understand it!” The necklace (supposedly) contained this cryptogram, which people have been trying to decipher ever since. The will of fellow pirate Bernardin Nageon de L’Estang (allegedly) refers to “considerable treasure … buried on my dear île de France” (now Mauritius), and the puzzle may or may not be related to carvings found in the rocks at Bel Ombre beach in the Seychelles by L’Estang’s descendant Rose Savy in 1923.
Does any of this add up to anything? Who knows? Nick Pelling has a good skeptical discussion here, including an interpretation of the cryptogram as a pigpen cipher.