Can refer to a hidden or lesser-known world, often with negative connotations.
El submundo de las leyendas urbanas me fascina y me intriga a la vez.
The underworld of urban legends fascinates and intrigues me at the same time.
El submundo de la ciudad está lleno de misterios y secretos por descubrir.
The underworld of the city is full of mysteries and secrets to uncover.