The recent discovery of another painting hidden beneath the early Picasso masterpiece, The Blue Room, is the latest in a number of finds made possible by X-ray technology that analyses the different layers of paint used by the artist.
Click on the masterpieces to see what lies beneath.
1. Picasso's Blue Room
It had long been suspected that another painting lay behind The Blue Room, one of the Spanish artist's key pieces from his "blue" period in the first few years of the 20th Century.
Reports say the idea was first raised in the 1950s, but it was not until 40 years later that X-ray technology revealed a fuzzy image beneath the famous portrait, which depicts a woman bathing.
Using infra-red technology, experts in the United States discovered a portrait of a bearded man wearing a jacket and bow tie and resting his hand on his cheek underneath the famous work.
Experts say it is likely the painter could not afford to buy a new canvas when he painted The Blue Room in 1901.
2. Van Gogh's Patch of Grass
Hidden beneath one of Vincent Van Gogh's paintings in his signature vivid style is a much more restrained portrait of what is thought to be a Dutch peasant woman painted by the artist in the early part of his career, when he lived in Nuenen in the Netherlands.
In late 1884, Van Gogh painted a series of heads of peasant models in their homes as a means of training his control over colour and light, and the woman discovered beneath Patch of Grass is likely part of this series.
What is significant about the discovery of the face is it shows the rapid evolution of his style and use of colour, as the two layers are estimated to be only painted two-and-a-half years apart, with Patch of Grass dated to 1887.
The painting was one of the first to be analysed with an X-ray technique pioneered by Dutch scientists Joris Dik and Koen Janssens, that can accurately determine the original pigments of the hidden painting.
3. Goya's Portrait of Don Ramón Satué
Buried beneath Goya's iconic portrait of his friend, Spanish judge Ramón Satué, is a much more formal portrait of a man in uniform.
Based on the uniform decorations, experts say the portrait could be of Napoleon Bonaparte's brother Joseph, who was put on the Spanish throne after Napoleon's conquest of the country in 1808.
Joseph Bonaparte was driven from Spain in 1813, and while Goya remained behind to become official court painter, it would have been dangerous for the artist to be in possession of such a work, perhaps explaining his decision to paint over it.
The vivid colours in the image beneath are a recreation of the original pigments by the Rijkmuseum's conservator.
4. Anthonissen's View of Scheveningen Sands (The beached whale)
While not a fully hidden painting, a conservation student at Cambridge University's Fitzwilliam Museum made a strange discovery while cleaning the 17th Century Dutch seascape: a man floating above the ocean.
This discovery sparked a detailed cleaning and restoration that reveal a beached whale, completely changing the composition of what had been a sedate sea-side image.
The painting dates from a time when there was a strong interest in whales, with records from the time showing many beachings on the Dutch coast. The whale was likely covered up by a later owner who did not like it.
Student Shan Kuang says the discovery shows how the attitudes to art have changed, as in the past it was normal to alter, or even cut up a painting.
5. Seurat's Young Woman Powdering Herself
In Georges Seurat's portrait of his mistress, Madeleine Knobloch, lies a hidden self-portrait that he covered over to protect his dignity.
The painting shows Ms Knobloch, an artist's model with whom he had a secret affair, applying make-up.
Seurat had originally painted his own likeness into the work, but the story goes that a friend - unaware of his relationship with Ms Knobloch - commented that it looked comical.
In response an embarrassed Seurat painted over it, thus depriving the world of his only self-portrait.
6. Allori's Isabella de Medici
The Carnegie Museum of Art in Pittsburgh was sceptical of the authenticity of a supposed 16th century painting of an Italian noblewoman, and sent it to conservators to confirm it was a fake, most likely from the 19th century.
However close inspection of the cracks on the surface led conservators to X-ray the painting and reveal a less-than-stunning secret.
Beneath the portrait of the comely Victorian-style maiden was an older woman with hands the 19th century restorer felt were too large.
Conservators speculated the Renaissance work was repainted in Britain to make it easier to sell.
7. Last but not least: Elias Garcia Martinez' Behold the man
While this remarkable "restoration" is not an exercise in stripping back layers of paint, we just couldn't leave it out.
Ecce Homo - or 'Behold the Man' - refers to a style of painting that depicts Jesus crowned with thorns, and this version is found in the Santa de la Misericordia, a church in Borja, Spain.
Elderly parishioner Cecilia Gimenez said she tried to fix the damage to the 19th Century fresco with approval from the church's priest.
Her efforts were met with horror by conservators, but sparked a social media frenzy and a flood of tourists to the town.
