5 Leonardo da Vinci's "Mona Lisa"
Possibly the most famous painting in the world, the "Mona Lisa" is also one of the most mysterious. Obviously it's a portrait of a woman, but which woman? Theories range from Lisa del Giocondo, the wife of a wealthy Florentine merchant, to the duchess of Milan or even Leonardo himself. Another major mystery is the "Mona Lisa's" smile: Sometimes it looks like she's smiling, but sometimes not, making her expression enigmatic. Nor does anyone know what she's smiling at or why, although this question has inspired other artists and poets for centuries.
4 Giorgio de Chirico's "The Child's Brain"
In the 1910s, de Chirico started exhibiting "metaphysical paintings," mysterious works based on dreams, mythology and philosophy that greatly influenced the surrealists. "The Child's Brain" is one of de Chirico's most famous metaphysical pieces and is often interpreted as a Freudian representation of the father figure. This is completely incorrect, a misunderstanding wrought by André Breton, who renamed the painting and applied his own interest in psychology to the interpretation. Although no one knows the exact meaning, the painting is probably about poetic revelation and the figure is de Chirico himself.
3 The Venus of Willendorf
One of the oldest known pieces of art in the world, dating from about 25,000 years ago, this small statue of a nude woman was discovered near Willendorf, Austria, hence the second part of its name. The first part of the statue's name, "venus," implies the woman is a goddess figure. In fact, no one knows if this nude figure was meant to represent a goddess, an actual prehistoric woman, a priestess or a queen. Even the pattern on the figure's head is something of a mystery: Does it represent hair or a hat? Unless new archaeological evidence comes to light, the Venus of Willendorf is likely to remain an enigma.
2 Francisco de Goya's "Black Paintings"
Goya is considered the first modern artist, yet his life is something of an enigma—very few things are known about him for certain. And the most mysterious aspect of his life and work is the so-called "Black Paintings," a collection of 14 paintings Goya painted on the walls of his home in Madrid between 1819 and 1824. They're called "black" both because of their coloring and subjects. No one knows why Goya decorated his house in this way or what the paintings are supposed to mean. In fact, some doubt Goya even painted them.
1 Giorgione's "The Tempest"
Venetian painter Giorgione's most well-known painting is also his most mysterious: "The Tempest" from around 1506. The painting itself is an evocative landscape with a city and storm in the distance, featuring male and female figures in the foreground. Because the symbols Giorgione used were non-standard, it's completely unknown what's going on in this piece. Some theories are that "The Tempest" is an allegory of conflict in Padua, an illustration of a Greek myth or that it depicts Adam and Eve, but in fact no one knows what the piece means and probably never will.