The underworld has two main meanings:

1. In ancient mythology and religion, the underworld is the place where souls go after death. The ancient Greeks believed in an underworld ruled by the god Hades, where spirits journeyed across the river Styx. The ancient Egyptians imagined their own underworld, called the Duat, where souls faced challenges before reaching the afterlife. Different cultures created their own versions: some dark and gloomy, others more complex, with different regions for different types of souls. When you read myths about heroes like Odysseus or Orpheus descending into the underworld, they're making dangerous journeys to the land of the dead.
2. The criminal underworld refers to the secret, organized world of criminals and illegal activity in a society. This includes gangs, smugglers, thieves, and others who operate outside the law, often with their own rules and hierarchies. Police detectives work to infiltrate the underworld and bring criminals to justice. The word suggests something hidden beneath normal society, like a shadow world operating in secret. When a news report mentions “underworld figures,” it means people involved in organized crime. The criminal underworld thrives on secrecy, since exposure to law enforcement can mean arrests and prosecution.