Aliens and UFOs are two of the most debated and pondered things in human history. Our universe contains two trillion galaxies aside from our own, making it impossible to prove if life exists on another planet in another galaxy. Who's to say if aliens visited Earth centuries ago? Is it possible they left messages for us through art? There are endless possibilities, but for now, here's a list of classical paintings that may (or may not) have UFOs or aliens hidden inside.