underground organization; clandestine group
Refers to a group operating secretly, often for political, criminal, or resistance purposes.
The police arrested members of the underground organization.
He was involved with an anti-government underground organization.
秘密結社 emphasizes a secret society with rituals or oaths, while 地下組織 is broader and often implies illegal or covert political activity.
Compound of 地下 (underground) and 組織 (organization).