underground

underground adjective, adverb uk/ˈʌndəɡraʊnd/ us underground adjective, adverb
(POLITICAL ACTIVITY)

Underground political activities are secret and illegal.

подпольный, подпольно
an underground political organization

UNDERGROUND noun

[ no plural ] UK uk/ˈʌndəɡraʊnd/ (US subway)
A2 a system of trains that is built under a city

метро
the London Underground