Open English Wordnet
Nouns
(n)metro, tube, underground, subway system, subwayan 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'”
Adjectives
(s)belowground, undergroundunder the level of the ground“belowground storage areas”, “underground caverns”
(s)clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, undercover, undergroundconducted 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”