adj. 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.