Definition of underground:
underpass, partizan, covered, hole-and-corner, surface, immunity, tube-shaped structure, subterranean, subway system, subsurface, irregular, impedance, resistivity, resistor, resistance, radical, cloak-and-dagger, thermionic tube, hypogeous, subterrestrial, under, surreptitious, pipe, clandestine, covert, vacuum tube, partisan, guerrilla, on the quiet, thermionic valve, electric resistance, ohmic resistance, hush-hush, experimental, secret, guerilla, metro, hypogean, subway, thermionic vacuum tube, opposition, electron tube, hypogeal, buried, avant-garde, belowground, unusual, undercover, hugger-mugger, tubing, tube, electrical resistance.
- secret (part of speech: adjective)
- "Studies of American Fungi. Mushrooms, Edible, Poisonous, etc.", George Francis Atkinson.
- "The Promised Land", Mary Antin.
- "The Blue Moon", Laurence Housman.