London ( CNN ) -- Dan Brown 's `` The Da Vinci Code '' encouraged amateur `` symbologists '' everywhere to scan their favorite paintings for secret codes -- but the practice has been going on for centuries . From the inscrutable prehistoric cave drawings inside the Chauvet Caves in Southern France to the abstract paintings of Jackson Pollock , artists have kept the intentions and significance of their work close to their chests . Famously opaque paintings include Bronzino 's 1545 work `` An Allegory with Venus and Cupid , '' which contains allegorical figures even scholars are n't able to decipher . Still , historians often discover hidden images , symbols and texts in works of art . CNN World 's Treasures selects examples of such discoveries -- some reputable , others outlandish . 'Twentieth Scene of the Life of St. Francis , ' Giotto di Bondone . The latest example is the discovery of a profile of the devil , secreted in this fresco by thirteenth-century Italian artist Giotto di Bondone at the Basilica of Saint Francis in Assisi . Italian art historian Chiara Frugoni discovered the hook-nosed , smirking devil hidden in the swirls of a cloud . She believes it refers to the Medieval belief that devils would appear in the clouds to arrest the soul 's ascent into heaven . 'Mona Lisa , ' Leonardo da Vinci . Leonardo da Vinci 's painting of an enigmatic smiling woman has been a source of intrigue for years . In 2010 the president of Italy 's National Committee for Cultural Heritage , Silvano Vinceti , was convinced he had discovered letters and numbers embedded in the eyes of the portrait , which he found after digitally magnifying it . Initially , Vinceti 's discovery of the letters L and V was taken as proof of the well-worn theory that the `` Mona Lisa '' is in fact a portrait of da Vinci himself . Vinceti then arrived at the conclusion that it was a painting of one of da Vinci 's male associates , though other art historians have been quick to debunk the idea . 'Battle of Marciano , ' Giorgio Vasari . This fresco in the Hall of Five Hundred in Florence 's Palazzo Vecchio is believed to hold the key to a long-lost work by Leonardo da Vinci . Art diagnostic expert Maurizio Seracini is convinced that da Vinci 's fresco `` The Battle of Anghiari '' lies behind `` The Battle of Marciano . '' Writer and painter Vasari , a great admirer of da Vinci 's , wrote about him beginning work on the fresco but abandoning it part of the way through due to technical problems . On a tiny painted green flag in Vasari 's fresco are the words `` Cerca , trova '' -- in English , `` seek and you shall find . '' Seracini is hoping to use forensic techniques to get behind Vasari 's fresco and see whether `` The Battle of Anghiari '' really does lie behind it . 'Sistine Chapel , ' Michelangelo . Michelangelo 's sublime Sistine Chapel at the Vatican in Rome received a thorough restoration and cleaning towards the end of the twentieth century , revealing this Renaissance masterwork in all its colorful , complex glory . But according to Roy Doliner , co-author of the book `` The Sistine Secrets : Michelangelo 's Forbidden Messages in the Heart of the Vatican , '' the restoration also revealed some seditious and previously unnoticed communications . One discovery was of a painting of a young Jewish man , Amminadab , wearing the sign of persecution prevalent at the time in Italy on his clothing , and apparently making a rude hand-gesture directly over where the Pope 's throne would have been . Another discovery was of two Jews in the inner circle of the elect in the towering `` Last Judgement '' -- `` total blasphemy for the 1500s , '' according to Doliner . It is his belief that Michelangelo was secretly preaching ideas of acceptance during a time of religious intolerance . 'The Last Supper , ' Leonardo da Vinci . One of the more outlandish theories about this work ( which featured as the central clue in the plot of `` The Da Vinci Code '' ) is that the undulating composition of the disciples ' hands and the bread rolls on the table correspond to musical notes on a stave . Strung together , the tune supposedly resembles a requiem -- a song for the dead . Leonardo was a talented musician in addition to his other great talents ; perhaps a musical painting is n't entirely out of the question ? 'The Supper at Emmaus , ' Caravaggio . Caravaggio 's 1601 painting `` The Supper at Emmaus '' may not have a hidden message that we know of but it does contains subtle symbols hinting at the identity of the mysterious , beardless man at the table . An important clue is the shadow of a fishtail coming off the bowl of fruit magically hovering at the edge of the table , a just-visible sign that the man at the table is Christ , the fisher of men . Another is the way all lines , like those in `` The Last Supper , '' point to the beardless man at the head of the table -- Christ , in disguise .
Hidden and hidden messages often appear in historical documents -- some credible , others less so . Face of the devil recently appeared in a Giotto fresco in a church in Italy . Art historians believe they have found messages in works by Leonardo da Vinci and Michelangelo . Forensic art historians hunting lost Leonardo work based on ` ` clue '' in Vasari fresco , among others .