1. the world of organized crime and criminals
2. in mythology and religion, the place where the spirits of the dead are believed to go

Examples

• The detective spent years investigating the city’s underworld.
• He was drawn into the dangerous underworld of illegal gambling.
• The novel explores connections between politicians and the criminal underworld.
• In Greek mythology, Hades ruled the underworld.
• Ancient cultures had various beliefs about the underworld and life after death.
• The hero’s journey took him through the underworld to rescue his beloved.