underground

(noun) a secret group organized to overthrow a government or occupation force. Synonyms: resistance.

(noun) an electric railway operating below the surface of the ground (usually in a city). Synonyms: metro, tube, subway system, subway. Example: "In paris the subway system is called the `metro' and in london it is called the `tube' or the `underground'."

(adj) under the level of the ground. Synonyms: belowground. Examples: "Belowground storage areas." "Underground caverns."

(adj) conducted with or marked by hidden aims or methods. Synonyms: clandestine, cloak-and-dagger, hole-and-corner(prenominal), hugger-mugger, hush-hush, secret, surreptitious, undercover. Examples: "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."

(adv) in or into hiding or secret operation. Example: "The organization was driven underground."

(adv) beneath the surface of the earth. Example: "Water flowing underground."