underground

[1]	resistance
[2]	metro , tube , subway system , subway
[3]	tube , metro , underground railway , (US) subway
[4]	resistance , partisans/partizans , freedom fighters , (in France) Maquis , insurgents , seditionaries/seditionists , insurrectionists , guerrillas/guerillas , extremists , revolutionaries , fifth-columnists , fifth column , saboteurs , subversives

adj.

[1]	subterranean , buried , below-ground , sunken , covered
[2]	secret , clandestine , concealed , hidden , covert , undercover , surreptitious , stealthy , private
[3]	alternative , belowground , radical , experimental , avant-garde , nonconformist , revolutionary
[4]	clandestine , cloak-and-dagger , hole-and-corner , hugger-mugger , hush-hush , secret , surreptitious , undercover