From the Champs-Elysées to Saint Germain des Près, the Bastille to posh Passy, Paris is the undisputed capital of girl watching. And with their pert presence, sense of style, and fashion flair, the ladies in question are indeed well worth a look. The city has even turned it into a spectator sport of sorts — besides fiddling with cell phones, what else are all those sidewalk cafés for? This year’s long simmering summer was an exceptionally good vintage, café tables overflowing to the curbs with goggle-eyed male patrons looking their fill.

But la Parisienne, for all her many assets and attributes, is not actually the most ogled woman in the city. That title goes to an aloof Italian beauty who neither flounces by nor makes with the saucy repartee. She merely gazes back with a subtle, enigmatic smile.

For a look at her, you have to start an epic journey in a cavernous crypt beneath a glass pyramid. You climb worn, crowded stone stairs of an ancient palace now known, for reasons no one can remember, as the Louvre. On the way you pass a cast of extras including an Italian slave, an ancient Greek warrior, and a carelessly draped winged lady. On the second floor you invariably come across a horde jockeying and elbowing as close as they can get to a bulletproof, air-conditioned showcase. If you can squeeze your way in, you are entitled to a harried look at Lisa Gherardini, a.k.a. Mona [a variation of Madonna, Lady] Lisa, the wife of a wealthy Renaissance Florentine merchant, Franceso del Giocondo.

Louvre officials estimate that fully 80 percent of the museum’s 6.6 million annual visitors come mainly to look at Leonardo da Vinci’s 500-year-old portrait. The question is, why? One answer is that like most celebrities, Mona Lisa is famous for being famous. Another is that they come to see the cultural archetype that has provoked more arcane analysis, gross imitations, and crass commercialization than just about any other object in the world.

If there was ever any doubt about her world-class status, that was laid to rest in January 1963, when Mona Lisa arrived in America. President John F. Kennedy and an evening-gowned Jacqueline formally welcomed her to Washington’s National Gallery of Art, where white-gloved U.S. Marines stood guard around the clock and crowds waited for hours. It was the same mob scene later at New York’s Metropolitan Museum. In all, more than 1.5 million Americans looked over Mona Lisa.

What they saw is defined by that tight-lipped smile. After looking carefully, the art critic Bernard Berenson considered that Leonardo’s subtle sfumato technique of modeling light and shade reached its apex here, carrying “facial expression perilously close to the brink of the endurable.” For centuries many an artist has tried to equal it. One, the mid-19th-century French artist Luc Maspero, threw himself out the window of his Paris room, leaving a farewell note: “For years I have grappled desperately with her smile. I prefer to die.”

These days we don’t need suicidal artists to analyze the dreamy, diaphanous atmosphere that seems to envelop her. We can use an X-ray fluorescence spectroscope, as technicians at the Louvre have recently done. With this they were able to detect dozens of layers of translucent glaze, each only one or two micrometers thick, that give her face a sense of depth and reality. Leonardo’s sfumato technique was well known before, but this is the first scientific explanation of exactly how he did it.

But beyond mere technique, the question remains of what it all means. The Marquis de Sade, for one, found Mona Lisa full of “seduction and devoted tenderness,” and “the very essence of femininity,” though given his tastes in women one wonders exactly what he meant. Walter Pater, leader of the 19th-century English Aestheticism movement, was even more overwrought at the sight of her. “She is older than the rocks among which she sits,” he swooned, “like the vampire, she has been dead many times, and learned the secrets of the grave; and has been a diver in deep seas.”

Sigmund Freud, too, went into raptures. Terming Leonardo an obsessive neurotic in Leonardo da Vinci: A Study in Psychosexuality, the Viennese supershrink decided that Mona Lisa’s expression could only resemble the mysterious smile of the artist’s mother: “This picture contains the synthesis of the history of Leonardo’s childhood.” As for Mona Lisa herself, he proclaimed her nothing less than “the most perfect representation of the contrasts dominating the love-life of woman, namely reserve and seduction, most submissive tenderness and the indifferent craving, which confront man as a strange and consuming sensuality.”

OVER AT THE LOUVRE, they have a more playful view. What if a pun lay at the heart of Mona Lisa? After all, Giocondo in Italian, like Joconde in French, means cheerful, merry, joyous, as does “jocund” in English. Leonardo had already played with a sitter’s name by putting a juniper bush in his portrait of Ginevra (related to “juniper” in Italian) de Benci that hangs in the National Gallery of Art. “He was punning on Mona Lisa’s married name when he gave her a subtle smile in La Joconde,” a curator of 16th-century French and Italian painting at the Louvre once told me, using the usual French term for the painting. “He made it emblematic of her. What we really have here is an idea, more than a realistic portrait, the idea of a smile expressed in a painting.” She added with a verbal shudder, “That picture always makes me feel uneasy when I look at it.”

Today’s ideas on art are more down-to-earth. Like, how much is it worth? King Francis I added Mona Lisa to his royal collections for 4,000 gold écus, or about $128,700, after Leonardo’s death in 1519 at his chateau of Amboise. Louvre officials say simply that Mona Lisa’s monetary value is inestimable. In 1911, however, it was precious but not yet such an icon that it couldn’t be sold. That made it worth stealing.

The biggest art heist in history — Time recently ranked it one of the most famous crimes of the last 100 years — occurred that year, when an Italian laborer named Vincenzo Peruggia walked out the door with it one evening. Peruggia, who had worked at the Louvre, was put up to the job by an Argentine con man named Eduardo de Valfierno, who had a skilled art forger knock off six copies. Valfierno then sold the copies for the equivalent today of $67 million. When Peruggia naively proposed the original to a Florence art dealer he was promptly pinched. Mona Lisa, undamaged, returned to France on December 31, 1913, riding like royalty in a special compartment of the Milan-Paris express, escorted by a squadron of policemen, politicians, museum officials, and artists.

The damage was to the blind veneration of Mona Lisa. Somehow the caper and its irreverent press coverage rubbed off some of her mystique. The age of Giocondoclasm had begun.

Suddenly the public couldn’t get enough of jokey Giocondiana. One postcard showed a grinning, toothy Mona Lisa thumbing her nose at the public and saying, “I’m off to see my Leonardo.” Another postcard, after the return, showed her holding a baby with Peruggia’s picture in the background, as if she’d been on a romantic escapade.

With irreverence and reaction against “bourgeois” values the new order of the day, the painting became the ideal target for desperately modern iconoclastic artists. Marcel Duchamp, leader of the Dada anti-art movement, summed up the new zeitgeist in 1919. Taking a standard postcard reproduction, he naughtily brushed in a pointy mustache and goatee on the sacred face. Now it looks like no more than a childish prank. But the uptight Art Establishment was shocked, shocked.

TODAY MONA LISA is the ultimate in kitsch. Artists vie to see who can do the most outrageous parody, advertising studious labor to come up with the funniest way to sell everything from aperitifs to airlines, golf clubs to strips that hold your nasal passages open. Collectors have catalogued nearly 400 advertising uses of the image and counting, along with at least 61 products called Mona Lisa in 14 countries, from rosé wine and chocolate to cigars, cheese, hairpins, potatoes, corsets, and beer.

Though he’s no high-flying art critic, Jean Margat has his own answer to the painting’s mythic status. A retired geologist, Margat from his home near Orleans presides over The Friends of Mona Lisa, a club of serious collectors of Giocondiana. His own vast, house-filling collection includes Mona Lisa T-shirts, posters, ballpoint pens, coffee mugs, drink coasters, condoms, panty hose, clocks, matchbooks, and thimbles bearing The Face. “There’s no way you can get away from it today,” he told me cheerfully. “Mona Lisa has an enormous recognition factor, like the Eiffel Tower or a top model. For better or worse, I’m afraid she still symbolizes Western art.”

Well, it could be worse. She’s really pretty good-looking.