Library of Congress An 1853 print by famous Japanese artist Toyokuni Utagawa shows a ninja sneaking up behind a musician. Ninjas are everywhere in our popular culture - they slice up mobsters in movies and fruits on your iPhone. We seem to think it makes sense to compare them to pirates and then think of them fondly as adolescent mutant turtles living in a New York sewer with a kimono-clad rat. But who are the ninja, really? That's what John Man, a British travel writer and historian, set out to explore in his new book Ninja: 1,000 Years of the Shadow Warrior. He spoke to TIME about the fighters behind the legend. PHOTOS: Happy Ninja Day: The Top 5 Pop Culture Ninjas Next #1 - The Smartest Ninjas Didn't Wear Black