Isleworth Mona Lisa
According to Leonardo's early biographer Giorgio Vasari, Leonardo had started to paint "Mona Lisa" in 1503, but "left it unfinished". However, a fully finished painting of a "certain Florentine lady" surfaced again in 1517, shortly before Leonardo's death and in his private possession. The latter painting almost certainly is the one that now hangs in the Louvre. Based on this contradiction, supporters of the authenticity of the "Isleworth Mona Lisa" claim it is the unfinished "Mona Lisa", made at least partially by Leonardo, and the Louvre "Mona Lisa" a later version of it, made by Leonardo for his own use.
Mona Lisa
In December 2015, it was reported that French scientist Pascal Cotte had found a hidden portrait underneath the surface of the painting using reflective light technology. The portrait is an underlying image of a model looking off to the side. Having been given access to the painting by Louvre in 2004, Cotte spent ten years using layer amplification methods to study the painting. According to Cotte, the underlying image is Leonardo's original Mona Lisa.