Definition of underground :
1. Being below the surface of the ground; as, an underground story or apartment.
2. Beneath the surface of the earth.
3. Done or occurring out of sight; secret.
4. The place or space beneath the surface of the ground; subterranean space.
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)
- It occurs from July to October about the bases of old stumps, dead trees, or from underground roots. - "Studies of American Fungi. Mushrooms, Edible, Poisonous, etc.", George Francis Atkinson.
- Polotzk had no underground communication with the sea, save such as water naturally makes for itself. - "The Promised Land", Mary Antin.
- " Why," answered the gnome, " for three years your daughter must come and live with me underground, and by the end of the third year her skin will be changed into pure gold like ours. - "The Blue Moon", Laurence Housman.