[n] electric underground railway
[n] a secret group organized to overthrow a government or occupation force
[adv] beneath the surface of the earth; "water flowing underground"
[adv] in or into hiding or secret operation; "the organization was driven underground"
[adj] used of independent armed resistance forces; "guerrilla warfare"; "partisan forces"
[adj] conducted with or marked by hidden aims or methods; "clandestine intelligence operations"; "cloak-and-dagger activities behind enemy lines"; "hole-and-corner intrigue"; "secret missions"; "a secret agent"; "secret sales of arms"; "surreptitious mobilization of troops"; "an undercover investigation"; "underground resistance"
[adj] under the level of the ground; "belowground storage areas"; "underground caverns"
Synonyms
belowground, clandestine, cloak-and-dagger, covert, guerilla(a), guerrilla(a), hole-and-corner(a), hugger-mugger, hush-hush, irregular, metro, on the quiet(p), partisan, partizan, resistance, secret, subsurface, subway, surreptitious, tube, undercover