2. (noun)metro, tube, underground, subway system, subway an electricrailway 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'"
3. (adj)belowground, underground under the level of the ground "belowground storage areas"; "underground caverns"
4. (adj)clandestine, cloak-and-dagger, hole-and-corner(a), hugger-mugger, hush-hush, secret, surreptitious, undercover, underground 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"
5. (adverb)underground in or into hiding or secretoperation "the organization was driven underground"
6. (adverb)underground beneath the surface of the earth "water flowing underground"
Definition of 'underground'
