The underworld in a city is the organized crime there and the people who are involved in it.
例句
...a Spanish Harlem underworld of gangs, drugs and violence...充斥着犯罪团伙、毒品和暴力的西班牙哈莱姆区黑社会Some claim that she still has connections to the criminal underworld.有人称她仍与黑社会犯罪势力有来往。
N-SING
地狱；阴间；冥府
In many ancient religions and legends, the underworld is a place under the earth's surface where people go after they die.
例句
...Persephone, goddess of the underworld.冥后珀耳塞福涅
英汉词典释义
n.
下流社会;黑社会
The central character of the novel is a sort of underworld figure.这部小说的中心角色是个黑社会人物。