In the age of GPS and Google Maps, it is hard to believe that maps can include places that don’t exist. But author Malachy Tallack argues that maps are as much “a cartography of the mind” as they are a way to figure out where we are. In his new book, The Un-Discovered Islands, Tallack takes readers on a journey to imaginary places—mythic islands, mapmakers’ mistakes, mirages, and outright hoaxes. [Find out how explorer Robert Peary discovered a continent that wasn’t there.]

When National Geographic caught up with Tallack on a Greek island (a real one), he explained why some islands blur the line between life and death; how others have moved about on the maps; and why