The underworld is the world of organised crime, or the land of the dead. The criminal underworld, underworld figures, a journey to the underworld.
The crime sense is the everyday one and always takes the, since it is treated as a single place. The mythological sense is where the word started, and it survives in writing about Greek and Roman stories.
1noununderworldThe world of organised crime, or the land of the dead.
“Two names from the underworld were named in the report.”
underworld is a world.
Middle English
world
English
world
English, today
underworld
Built from
From under- + world. world comes from Middle English world. From Middle English world, from Old English weorold (“world”), from Proto-West Germanic weraldi, from Proto-Germanic weraldiz (“lifetime, human existence, world”) literally “age/era of man”, equivalent to wer + eld. Eclipsed non-native mounde, from monde, munde (“world”). 2
Etymology from Wiktionary, CC BY-SA