Thanks to a wonderful coincidence, Leonardo da Vinci may have invented the first stereoscopic image in history. According to researchers Claus-Christian Carbon and Vera M Hesslinger, the Mona Lisa can be seen in 3D thanks to another version that was painted side by side as Leonardo was making the original.
Historians used to believe that this version—which can be seen at the Museo del Prado in Madrid, Spain—was just another copy of the original, made after Leonard0's piece was created. However, X-ray images showed that El Prado's Mona Lisa was painted by an apprentice at the same time of the original. It also includes the same landscape—which was initially covered with black paint—and corrections made by Leonardo da Vinci.
Further analysis revealed differences: The body of La Gioconda was shifted in relations to the background, which is believed to be a flat canvas containing the landscape and positioned behind the model. Her body perspective was different too. You can clearly see how there are two points of view in the comparison GIF I made above, especially her body and the background.
This all makes sense, considering that the two artists were located at different positions, having a sightly different point of view:
The researchers found out that this binocular treatment resulted in two different paintings that, when combined, result in the stereoscopic image. In fact, they point out in their study that "the two [paintings] together might represent the first stereoscopic image in world history."