You've seen the graffiti, the tattoos, the headlines documenting their brutality. What is driving the rapid spread of the ultra-violent gang Mara Salvatrucha (MS-13)? National Geographic re-investigates MS-13, tracing the evolution and uncovering the truth about this notorious street gang.

Host Marsh Mokhtari travels the globe and brings viewers along as he undertakes some of the world's most exciting journeys. Marsh's thrilling and often treacherous routes take him through striking geography and give him an unique opportunity to meet fascinating people and experience an amazing array of cultures and lifestyles.