There have been hints for nearly a century that she has been lurking there, another woman hidden under the 19th-century brushstrokes of French master Edgar Degas' Portrait of a Woman.
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Now, thanks to the Australian Synchrotron, the mystery of the hidden painting looks to have been solved.
A non-invasive technique has revealed striking detail of this first portrait dreamed up by Degas.
The artistic and scientific sleuths believe that the woman obscured for so long is Emma Dobigny, a well-known artists' model of the time.
Scientists at the synchrotron, which is a type of particle accelerator, used a high-definition scanning technique to establish the nature of the paint lying beneath the finished work.
Just like an ink-jet printer moving back and forth over paper, the synchrotron's scanner moved across the painting, firing intense X-rays into the canvas.
Daryl Howard at the Australian Synchrotron told Fairfax Media: "Conventional X-ray methods hadn't given enough information to determine what was underneath the surface.
"The National Gallery of Victoria could tell there was a woman there, but nothing more could be determined."
It took 33 hours of scanning to retrieve the information needed to solve the puzzle.
"Ten years ago it would have taken 18 months," Dr Howard said.
The synchrotron uses a method called X-ray fluorescence elemental mapping. The beam fired at the canvas excites the individual metal atoms in the layers of paint, allowing the scientists to determine the composition and, in part, the colour of what lies beneath.
Using a radiation source produced by the synchrotron, Dr Howard, with curator David Thurrowgood and colleagues, produced maps of the painting using the X-ray fluorescence. These were then processed using a false-colour technique to produce the stunning portrait of what is likely Emma Dobigny.
Their results are published in Nature's Scientific Reports.
"We reconstructed the colours by examining the metals," said Dr Howard. "In some cases it's simple: cobalt means blue, mercury means vermillion. However with iron it could be many colours, so a certain amount of guesswork comes in.
"However, we can tell from looking at surrounding colours what is most likely."
Scientists and curators alike hope that this process will help with future analysis of paintings to help with restoration, determine forgeries and further understand the painting techniques of Old Masters like Degas.
Degas' painting from the late 1870s has hung in the National Gallery of Victoria since 1937. Its purchase at the time caused some controversy as it was not considered the best Degas available for purchase, but it was one of the cheaper available at the time.
