PARIS -- The Mona Lisa, Leonardo da Vinci's portrait of the unknown woman with the enigmatic smile, is sparking a new kind of mystery: What is causing the Renaissance masterpiece to deteriorate so quickly?
The thin, poplar panel on which the Mona Lisa is painted in oil has changed shape since conservation experts last evaluated it, the Louvre Museum said. Leonardo's masterwork - now nearly 500 years old - is inspected every one to two years.
The Louvre said the "state of conservation" of the most famous artwork in France's most famous museum "has aroused some worry."
The state-run Center for Research and Restoration of Museums of France will conduct a study to better determine what materials the painting is made of and evaluate its vulnerability to temperature changes.
Some seasoned visitors say they have noticed changes.
"We lived in Paris in 1962 and 1963 - she seemed brighter back then," said Enid Kushner, 74, a retired lawyer from Cleveland.
First-time visitor Kristy Vander Ploeg, 23, of Toronto, said: "I didn't expect it to look like that - it's a lot more faded than I thought it would be."
The painting has yellowed from layers of varnish applied over the centuries, but the Louvre has resisted pressure to touch it up. The last real work on the Mona Lisa dates to the mid 1950s, when experts removed several age spots.
The Louvre says the most recent analysis can be done without taking the painting out of the public eye. The Mona Lisa now has its own wall; next year it will get its own room.
For the museum's crown jewel, little is left to chance: The painting is housed in an air-conditioned glass case, and visitors are held back by a waist-high barrier.
The Mona Lisa is seen each year by nearly all the 6 million people who visit the Louvre. Just last year, it was on the cover of Dan Brown's best seller, "The Da Vinci Code," and figured in the plot.
On Monday, as usual, rubbernecking tourists peered at the painting, and camera flashes went off with the speed and frequency of a Paris fashion show.
Louvre spokeswoman Aggy Lerolle said the flash photography - which is "theoretically forbidden" at the museum - is not believed to be the cause of the painting's deterioration.
But some visitors, after hearing about the Mona Lisa's woes, felt a little guilty for using their flashes.
"It's our fault, I know," said Mikhail Kouzmenko, a Moscow executive, after a friend snapped him, smiling, in front of the painting. "It's bad for the picture, I know."
Most days, security guards and ushers keep the flow of tourists to a regular pace. Often lines to see the Mona Lisa stretch for dozens of yards.
Experts believe Leonardo painted the Mona Lisa in Italy over a long period beginning about 1505. The identity of the model is not known.
The work is clearly a survivor. During World War II, French authorities hid the painting in small towns to keep it out of the hands of German forces. In 1911, an Italian house painter stole the masterpiece, saying he planned to return it to Italy, but it resurfaced two years later.
The admiration that the painting has evoked has been attributed to fascination with Leonardo's genius; the painting's stunning realism and technique; the mystery of the Mona Lisa's identity; and the twists and turns in its history.
Some tourists say the Mona Lisa now is simply feeding off its own fame. And nearly everyone seems to have an opinion about it.
"Her popularity is just based on what has happened over the years," said Joanne Rosini, 40, an animal trainer from Brooklyn. "It's really a boring painting, she's just sitting there."
Her father-in-law, retired police officer Eugene Rosini, said he finds meaning the Mona Lisa's smile.
"It's a smile to tempt her lover," he said with a wry grin - acknowledging that the come-hither look might have worked on him.