Mona Lisa Underdrawing Researchers Discover a Hidden Drawing Behind 'Mona Lisa

No, this isn’t the plot of a Nicolas Cage movie. A special camera went deep into the famous painting
A special camera went deep into the famous painting's nooks and crannies. 

Study of the Prado Museum's copy of La Gioconda - Museo Nacional del Prado

Optoelectronic Applications: Nondestructive Testing - Laser-based instrumentation sheds new light on old art

Mona Lisa, USAM Wiki

A NEW WAY OF LOOKING AT THE MONA LISA, monalisa

Leonardo da Vinci's Mona Lisa – The Artistic Adventure of Mankind

The little white attic : Speculation about Mona Lisa

A NEW WAY OF LOOKING AT THE MONA LISA, monalisa

Leonardo's unidentified assistant—who painted the Prado's Mona Lisa—also copied Saint Anne and the Salvator Mundi, new research suggests

Mona Lisa Underdrawing Researchers Discover a Hidden Drawing Behind 'Mona Lisa