Meaning of “underground” - Learner’s Dictionary

underground

adjective, adverb uk us /ˈʌndəɡraʊnd/
UNDER SURFACE

B2 under the surface of the ground:

underground caves
an animal that lives underground
POLITICAL ACTIVITY

Underground political activities are secret and illegal:

an underground political organization

(Definition of “underground adjective, adverb” from the Cambridge Learner’s Dictionary © Cambridge University Press)