These are six of the toughest prisons in the world. Prisons so tough, even gang bosses and killers learn the meaning of fear. Across four continents, six different prisons, the series takes us beyond the gates, walls, barred windows and cells into an unknown world, a world we know exists, but a world nobody wants to think about. Imagine prisons so tough that even the hardest gang bosses and murderers learn the meaning of fear. This extraordinary series offers intimate portraits of crime and punishment set within the gates, walls, barred windows and cells of the world's toughest maximum security prisons. From Bolivia to Bulgaria, Dallas to Arizona, viewers meet hardened criminals with stories that shake them to the core.