The phrase 'criminal underworld' refers to a vast array of illegal activities and dealings that occur within a specific society. Synonyms for this term could include the 'shadow economy', 'underground network', 'black market', 'organized crime', 'illegal trade', 'secretive world' and the 'criminal underworld'. These terms all refer to a structure of illegal entities operating in secret, and making profits from illegal activities such as human trafficking, drug dealing, extortion, and gambling. These criminal organizations are notoriously difficult to infiltrate, and law enforcement agencies have to work tirelessly to dismantle them. It is important for society to recognize the existence of these groups and aim to tackle them to ensure safety and security in all communities.