adverb.beneath the surface of the earth

adverb.in or into hiding or secret operation

noun.a subterranean space or channel

noun.an underground city railway system

noun.a movement or group organized in strict secrecy among citizens especially in an occupied country for maintaining communications, popular solidarity, and concerted resistive action pending liberation

adjective.being, growing, operating, or situated below the surface of the ground

• an underground literary reputation
• the underground economy

adjective.conducted by secret means

• underground movies
• underground newspapers

adjective.existing outside the establishment

• an underground moviemaker
• an underground theater