Ancient Astronaut Theory has existed in popular culture for nearly 50 years, asking the question whether extraterrestrials visited Earth in the remote past. All around the world there are examples of physical evidence that suggest humans are not alone in the universe. But has this evidence been left behind for a purpose? Is it in fact a series of clues, or Alien Code, that we are meant to decipher? Ancient Astronaut theorist Giorgio A. Tsoukalos investigates unexplained mysteries from every corner of the world. The evidence shows that many ancient sites, and even modern ones like Roswell, all seem to be associated with legends and reports of someone or something coming down from the sky. Tsoukalos' research demonstrates that many of these sites share a common symbol or shape–concentric circles. What is the significance of these designs, and is it possible they contain a coded, mathematical message left by alien visitors in the remote past?