On Friday morning, The Guardian reported that a centuries-old portrait of Mary, Queen of Scots was recently discovered hiding underneath a portrait of Sir John Maitland, former lord chancellor of Scotland. The Queen's image—which, for almost 450 years, had been thought to be lost—had been hanging in not-so-plain sight on the wall of a historic London home. According to the outlet, "Her portrait may have been considered dangerous, left unfinished, and then overpainted by the nervous artist, in the political turmoil after she was executed in 1587." Now the ghostly image will go on display at the Scottish National Portrait Gallery. The story, while exciting for both art and history buffs, is not unusual.

Just underneath the surface of many paintings, both famous and obscure, is another hidden painting that could have been. Sometimes, these ghostly images are apparent to the naked eye if you look closely enough. More often, they are revealed by restoration processes, x-rays, and careful investigation by art historians and preservation specialists. (In the case of Mary, Queen of Scots, it was an x-ray that found it.)

In some cases, scandal forced artists to correct controversial details; in others, the artist simply changed his or her mind. During lean times, some artists resorted to painting over less satisfactory or unfinished work because they could not afford new canvas.

Instances of painterly corrections which expose previous versions of the design are referred to as pentimenti, from the Italian “to repent,” essentially because the artist has “repented” for a choice made earlier in the creative process. A pentimento can be, for example, a change in the position of a hand, the enlargement of a tablecloth, or the reduction of the size of a hat. Small pentimenti are everywhere in paintings, and can be more common among schools of painters who had workshops and assistants. The idiosyncrasies of pentimenti have even been used to identify lost works by great painters such as Leonardo da Vinci.

Whatever the circumstances, thousands of paintings contain fascinating omissions, fixes, and shrewd substitutions.

1. THE DISAPPEARING BUST OF THE KING OF ROME IN JEAN-AUGUSTE-DOMINIQUE INGRES’S PORTRAIT OF JACQUES MARQUET DE MONTBRETON DE NORVINS

This 1811-12 portrait of Napoleon’s Chief of Police in Rome by the French Neoclassical painter Jean-Auguste-Dominique Ingres features a shadowy trace of another face. Floating within the fabric of the lefthand curtain, the features of a completed bust of a child’s head can be seen, even with the naked eye. Art historians have also noticed something haphazard about the inclusion of the bust of Minerva on the right, which is so far out of frame that it seems like an afterthought.

Given the hasty and awkward omission of the figure on the left, it is thought to be a bust of the head of Napoleon’s son, who was dubbed the King of Rome. In 1814, Napoleon lost power, and association with him became—at the very least—unfashionable for a portrait painter. The coverup, which may not have been made by Ingres himself, is thought to be politically motivated.

2. THE HIDDEN WOMAN IN PABLO PICASSO’S THE OLD GUITARIST

During Pablo Picasso’s “Blue Period” (1901-1904), funds for art supplies were tight. Sometimes, when the artist was particularly strapped, he would substitute cardboard for canvas. When he had canvas, it was occasionally repurposed. One of the most well-known examples of the body of work Picasso created during this time, The Old Guitarist, turned out to have been painted over another figure.

If you have ever seen this painting in person, it is possible that you noticed what looked like another face, behind the bent neck of the guitarist. Although it is not clear who this hidden portrait is of, x-ray imaging has revealed a number of additional details. The woman is nursing a small child, and appears to be in some sort of pastoral setting as she is accompanied by a bull and a sheep.

3. THE BEARDED MAN BENEATH PABLO PICASSO’S THE BLUE ROOM

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Picasso’s 1901 Blue Period painting The Blue Room has more than its tone in common with The Old Guitarist. Recently, infrared imaging has uncovered another portrait underneath the room scene. The bearded man, who is in formal wear and can be seen to be wearing a number of rings on his fingers, reclines pensively when the painting is vertically oriented. He probably is, as was the woman beneath The Old Guitarist, another victim of Picasso’s canvas budget.

4. THE SALACIOUS STRAP IN JOHN SINGER SARGENT’S MADAME X

The portrait of “Madame X” is a familiar fixture at the Metropolitan Museum of Art, and somewhat of a style icon with her simple black dress, statuesque figure, and haughty expression. However, in its time this portrait was considered an unflattering, scandalous affront to decency, and it had a disastrous effect on the European career of its creator.

The woman in the portrait is Madame Pierre Gautreau, a New Orleans expatriate who was trying to make her mark on the European scene as a great beauty. The pallor of her skin, which is notable in the painting and prompted one contemporary critic to call her “cadaverish,” was achieved by ingesting arsenic wafers. She was known to heighten the effect by rouging her ears and deepening the color of her hair with henna.

Sargent, hoping to capture her at her most dramatic, selected her most striking black gown for her to wear. Most daringly, he painted her with one jewelled strap of her gown hanging from her shoulder.

When the portrait was first displayed in a salon exhibition, the outcry was instantaneous. Critics called the costume of the subject “flagrantly insufficient,” and Madame Pierre Gautreau’s humiliated family called for it to be taken out of the exhibition. Sargent, in a rare moment of self-doubt, took the painting and fashioned a properly placed strap on the now infamous Madame X’s shoulder.

5. THE DYE JOB IN WOMAN AT A WINDOW

At the National Gallery in London, the restoration process of an early 1500s painting of a woman at a window by an unknown artist uncovered a remarkable makeover. What museum workers had originally thought to be varnish imperfections in the woman’s hair turned out to be the blonde locks of the original figure showing through a subsequently applied layer of paint.

The blonde underneath the modest brunette is a far more interesting subject. Her gaze is more calculated, her expression more confusing, and her bodice obviously more detailed. At some point, she was painted over as a humble brunette, with a modest expression and unthreatening cleavage. Today, the painting has been restored to its original state, and the Renaissance woman can be seen clearly again at the National Gallery.

6. THE REAPPEARING WHALE IN HENDRICK VAN ANTHONISSEN’S BEACH SCENE

When this seventeenth-century Dutch painting was donated to the Fitzwilliam Museum, it appeared to be a simple beach scene. However, the conservator at the Hamilton Kerr Institute in charge of restoring it before exhibition at the Fitzwilliam Museum thought it odd that a large crowd appeared to have congregated by the water in the distance for no discernible reason.

A little cleaning uncovered a figure, apparently standing on the horizon. More cleaning revealed that the figure was, in fact, standing atop a beached whale which had been painstakingly painted over.

The reason for this coverup is thought to be a simple matter of interior decoration. The repainting is thought to have occurred during the 18th or 19th centuries. Paintings often served a decorative function, and were as much a part of a well-appointed living room as were chairs and rugs. It is entirely possible that a whale carcass was considered an unsavory image to have in a drawing room. The dead whale is now restored to its former glory, and will decay proudly in public view for years to come.