Leonardo da Vinci is esteemed for the hidden complexities in his work, some of which have even led to speculative fiction and modern conspiracy theories, such as portrayed in "The Da Vinci Code." Perhaps it should come as no surprise, then, that his most famous portrait, "The Mona Lisa," should hide some mysteries of its own.

French scientist Pascal Cotte claims to have identified a hidden portrait underneath the Mona Lisa, which had been painted over and transformed into the portrait we now recognize today, reports the BBC. Perhaps the biggest mystery of all: The hidden portrait does not display Mona Lisa's unforgettable enigmatic smile.

Cotte was given access to analyze the painting from the Louvre back in 2004. For the last 10 years, he has been using non-invasive techniques to scour the masterpiece for clues and details about its creation. A new pioneering technique called Layer Amplification Method (LAM), which works by "projecting a series of intense lights" on to the painting, has revealed the background portrait.

"We can now analyze exactly what is happening inside the layers of the paint and we can peel like an onion all the layers of the painting. We can reconstruct all the chronology of the creation of the painting," said Cotte.

The finding means that it is possible that the Mona Lisa we have come to know is not the real Mona Lisa. Aside from lacking a smile, the woman depicted in the hidden background portrait also lacks a direct gaze. Instead, she appears to be looking off to the side.

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Although some debate about the true identity of the Mona Lisa persists, most experts believe that she represents Lisa Gherardini, the wife of a silk merchant from Florence. But Cotte thinks his analysis could reignite the debate.

"The results shatter many myths and alter our vision of Leonardo's masterpiece forever," he said. "When I finished the reconstruction of Lisa Gherardini, I was in front of the portrait and she is totally different to Mona Lisa today. This is not the same woman."

Cotte also claims to have found two more images under the surface of the painting: a deeper, shadowy outline of a portrait with a larger head, nose and hands, but smaller lips, as well as another Madonna-style image with etchings of a pearl headdress.

Some art historians are skeptical that these background images represent different portraits, however. Rather, they claim, these earlier etchings are probably rough drafts, which da Vinci painted over and edited as part of a natural artistic process.

"I do not think there are these discrete stages which represent different portraits," said Martin Kemp, Emeritus Professor of the History of Art at the University of Oxford. "I see it as more or less a continuous process of evolution. I am absolutely convinced that the Mona Lisa is Lisa."

Regardless of the true identity of the hidden background portrait, Cotte's method gives researchers unprecedented access to how exactly da Vinci proceeded in creating his masterpiece. It's almost as if researchers can now retrace each stroke of the brush, to reveal Leonardo's artistic choices as he made them. It's remarkable detective work.

The findings will be presented in a documentary, "Secrets of the Mona Lisa," set to air on the BBC.