How many times have you seen images of Leonardo da Vinci's Mona Lisa? 10? 100? Either way, you've probably never noticed the artist's initials written in the subject's right eye. And that's just one of the work's many mysterious qualities, including the number 72 painted into the bridge behind Mona Lisa. Da Vinci wasn't the only artist to hide symbols and meanings into his work. Botticelli's "Primavera" depicts more than 500 different plant species, including 150 different flower species. Michelangelo's "The Creation of Adam" shows God reaching out to Adam from a cloud that is shaped like a brain. As a teenager, Michelangelo dissected corpses in the church cemetery, which is where this accurate brain representation may have come from.