The noun underground has 2 senses (no senses from tagged texts)
1. underground, resistance -- (a secret group organized to overthrow a government or occupation force)
2. metro, tube, underground, subway system, subway -- (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'")
Overview of adj underground
The adj underground has 2 senses (first 1 from tagged texts)
1. (7) belowground, underground -- (under the level of the ground; "belowground storage areas"; "underground caverns")
2. clandestine, cloak-and-dagger, hole-and-corner, 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")
Overview of adv underground
The adv underground has 2 senses (no senses from tagged texts)
1. underground -- (in or into hiding or secret operation; "the organization was driven underground")
2. underground -- (beneath the surface of the earth; "water flowing underground")