underworld
n. countable B2 Upper Intermediate US //ˈəndɝˌwɝɫd// UK //ˈʌndəwˌɜːld// un·der·world
n. the place where dead people go in stories and myths, or the secret world of criminals.
n. the mythical abode of the dead, or the hidden world of organised crime and illegal activity.
In Greek myths, heroes often travel to the underworld.
The police spent years investigating the city's criminal underworld to find the gang leaders.
The poet describes a descent into a shadowy underworld where the spirits of the forgotten wander through eternal mist.
From under- + world.
Usage
Usually used with the definite article 'the' when referring to either the mythological or criminal sense.