What is the meaning of underground?

noun

• A secret group organized to overthrow a government or occupation force.
  • synonyms: resistance
• An electric railway operating below the surface of the ground (usually in a city. )
  • usage: "in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'"
  • synonyms: metro, tube, subway system, subway

adjective

• Under the level of the ground.
  • usage: "belowground storage areas"; "underground caverns"
  • synonyms: belowground
• Conducted with or marked by hidden aims or methods.
  • usage: "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"
  • synonyms: clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, undercover

adverb

• In or into hiding or secret operation.
  • usage: "the organization was driven underground"
• Beneath the surface of the earth.
  • usage: "water flowing underground"