Despite her monumental reputation, the Mona Lisa was a mere thirty-one inches by twenty-oneinches—smaller even than the posters of her sold in the Louvre gift shop. She hung on thenorthwest wall of the Salle des Etats behind a two-inch-thick pane of protective Plexiglas. Paintedon a poplar wood panel, her ethereal, mist-filled atmosphere was attributed to Da Vinci's masteryof the sfumato style, in which forms appear to evaporate into one another.

　　Since taking up residence in the Louvre, the Mona Lisa—or La Jaconde as they call her inFrance—had been stolen twice, most recently in 1911, when she disappeared from the Louvre's"satte impénétrable"—Le Salon Carre. Parisians wept in the streets and wrote newspaper articlesbegging the thieves for the painting's return. Two years later, the Mona Lisa was discovered hiddenin the false bottom of a trunk in a Florence hotel room.

　　Langdon, now having made it clear to Sophie that he had no intention of leaving, moved with heracross the Salle des Etats. The Mona Lisa was still twenty yards ahead when Sophie turned on theblack light, and the bluish crescent of penlight fanned out on the floor in front of them. She swungthe beam back and forth across the floor like a minesweeper, searching for any hint of luminescentink.

　　Walking beside her, Langdon was already feeling the tingle of anticipation that accompanied hisface-to-face reunions with great works of art. He strained to see beyond the cocoon of purplishlight emanating from the black light in Sophie's hand. To the left, the room's octagonal viewingdivan emerged, looking like a dark island on the empty sea of parquet.

　　Langdon could now begin to see the panel of dark glass on the wall. Behind it, he knew, in theconfines of her own private cell, hung the most celebrated painting in the world.

　　The Mona Lisa's status as the most famous piece of art in the world, Langdon knew, had nothing todo with her enigmatic smile. Nor was it due to the mysterious interpretations attributed her bymany art historians and conspiracy buffs. Quite simply, the Mona Lisa was famous becauseLeonardo da Vinci claimed she was his finest accomplishment. He carried the painting with himwhenever he traveled and, if asked why, would reply that he found it hard to part with his mostsublime expression of female beauty.

　　Even so, many art historians suspected Da Vinci's reverence for the Mona Lisa had nothing to dowith its artistic mastery. In actuality, the painting was a surprisingly ordinary sfumato portrait. DaVinci's veneration for this work, many claimed, stemmed from something far deeper: a hiddenmessage in the layers of paint. The Mona Lisa was, in fact, one of the world's most documentedinside jokes. The painting's well-documented collage of double entendres and playful allusions hadbeen revealed in most art history tomes, and yet, incredibly, the public at large still considered hersmile a great mystery.

　　No mystery at all, Langdon thought, moving forward and watching as the faint outline of thepainting began to take shape. No mystery at all.

　　Most recently Langdon had shared the Mona Lisa's secret with a rather unlikely group—a dozeninmates at the Essex County Penitentiary. Langdon's jail seminar was part of a Harvard outreachprogram attempting to bring education into the prison system—Culture for Convicts, as Langdon'scolleagues liked to call it.

　　Standing at an overhead projector in a darkened penitentiary library, Langdon had shared the MonaLisa's secret with the prisoners attending class, men whom he found surprisingly engaged—rough,but sharp. "You may notice," Langdon told them, walking up to the projected image of the MonaLisa on the library wall, "that the background behind her face is uneven." Langdon motioned to theglaring discrepancy. "Da Vinci painted the horizon line on the left significantly lower than theright.""He screwed it up?" one of the inmates asked.

　　Langdon chuckled. "No. Da Vinci didn't do that too often. Actually, this is a little trick Da Vinciplayed. By lowering the countryside on the left, Da Vinci made Mona Lisa look much larger fromthe left side than f............