Synonyms of underground

1. underground, resistance, revolutionary group: usage: a secret group organized to overthrow a government or occupation force
2. metro, tube, underground, subway system, subway, railway, railroad, railroad line, railway line, railway system: usage: an electric railway operating below the surface of the ground (usually in a city); "in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'"

1. belowground, underground, subsurface (vs. overhead) (vs. surface): usage: under the level of the ground; "belowground storage areas"; "underground caverns"
2. clandestine, cloak-and-dagger, hole-and-corner(prenominal), hugger-mugger, hush-hush, secret, surreptitious, undercover, underground, covert (vs. overt): usage: 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"

1. underground: usage: in or into hiding or secret operation; "the organization was driven underground"
2. underground: usage: beneath the surface of the earth; "water flowing underground"