The Oxford Thesaurus An A-Z Dictionary of Synonyms - undergroundadj.

1) subterranean, buried, below-ground, sunken, covered: They crept through the underground passage into the treasure room. 2) secret, clandestine, concealed, hidden, covert, undercover, surreptitious, stealthy, private: The secret service has a worldwide underground network of agents. 3) alternative, radical, experimental, avant-garde, nonconformist, revolutionary: Some underground newspapers that sprang up in the 1960s are still being published. --n. 4) tube, metro, underground railway, US subway: Two more stops on the underground and we'll be there. 5) resistance, partisans or partizans, freedom fighters, (in France) Maquis, insurgents, seditionaries or seditionists, insurrectionists, guerrillas or guerillas, extremists, revolutionaries; fifth-columnists, fifth column, saboteurs, subversives: The underground helped the family to escape to England. The government blames the underground for the bombings.