There may actually be one.
German visual scientists have found a Mona Lisa painting in the Museo del Prado in Madrid that may have been painted either by Da Vinci or by one of his students, visual scientists said.
The original Mona Lisa painting is in the Louvre Museum in Paris.
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The researchers from University of Mainz in Germany looked at so-called trajectories - or the paths from a distinctive point on the source such as the tip of Mona Lisa's nose.
It turned out that the real Mona Lisa and the Prado one were painted from slightly different perspectives.
They found that the two paintings form a stereoscopic pair. When viewed together, these create an impression of depth, a 3D image of the Mona Lisa.
This evidence "might indicate that Da Vinci did not only think about the 3D issue theoretically but in a very practical sense in terms of experiments", the researchers were quoted as saying in media reports.