London (CNN) -- Dan Brown's "The Da Vinci Code" encouraged amateur "symbologists" everywhere to scan their favorite paintings for secret codes -- but the practice has been going on for centuries.

From the inscrutable prehistoric cave drawings inside the Chauvet Caves in Southern France to the abstract paintings of Jackson Pollock, artists have kept the intentions and significance of their work close to their chests.

Famously opaque paintings include Bronzino's 1545 work "An Allegory with Venus and Cupid," which contains allegorical figures even scholars aren't able to decipher.

Still, historians often discover hidden images, symbols and texts in works of art.

CNN World's Treasures selects examples of such discoveries -- some reputable, others outlandish....