underground

a secret group organized to overthrow a government or occupation force

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'"

under the level of the ground; "belowground storage areas"; "underground caverns"

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"

in or into hiding or secret operation; "the organization was driven underground"

beneath the surface of the earth; "water flowing underground"