This is The Ambassadors (1533), the celebrated painting by Hans Holbein the Younger. It’s full of noteworthy symbols of exploration, but what’s that odd skewed element at the bottom?

If you view the canvas from a narrow angle, the image resolves into a skull:

This is an early example of anamorphic perspective, an invention of the early Renaissance. It’s thought that Holbein intended that the painting would be hung in a stairwell, when people ascending the stairs would view the image from the proper angle and get a gruesome surprise.

Why? That’s an unanswered question.