The "Criminal Underworld" refers to the network of illegal activities and organizations that operate outside the law. This includes various forms of crime such as drug trafficking, human trafficking, and organized crime. Groups involved often have a hierarchical structure, with leaders and members who engage in illicit activities for profit. These criminal organizations can vary in size and scope, from small local gangs to large international syndicates. They often use violence, corruption, and intimidation to maintain control and avoid law enforcement. The mafia, cartels, and gangs are examples of entities that operate within this hidden society.