Long before the internet made "the dress" go viral, Swiss painter Hans Holbein painted a mind-melting illusion. In his 1533 painting "The Ambassadors," commissioned for French diplomat Jean de Dinteville, there is something hiding at the bottom of the picture that you may not see at first glance.

The anamorphic skull is put in the painting as a memento mori, or a "reminder of death," and can only be fully seen from a certain angle.