n. 25B6; adjective
an underground car park : SUBTERRANEAN, buried, sunken, basement.
underground organizations : CLANDESTINE, secret, surreptitious, covert, undercover, closet, hole-and-corner, cloak-and-dagger, hugger-mugger, back-alley, hidden, sneaky, furtive; resistance, subversive; informal hush-hush.
the underground art scene : ALTERNATIVE, radical, revolutionary, unconventional, unorthodox, avant-garde, experimental, innovative.
25B6; adverb
the insects live underground : BELOW GROUND, in the earth.
the rebels went underground : INTO HIDING, into seclusion, undercover.
25B6; noun
he took the underground : UNDERGROUND RAILWAY, metro; N. Amer. subway; Brit. informal tube.
information from the French underground : RESISTANCE (MOVEMENT); partisans, guerrillas, freedom fighters; historical) Maquis.