Definition as Noun

• an electric railway operating below the surface of the ground (usually in a city)
• a secret group organized to overthrow a government or occupation force

Definition as Adjective

• under the level of the ground
• conducted with or marked by hidden aims or methods

Definition as Adverb

• in or into hiding or secret operation
• beneath the surface of the earth

• belowground

• clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, undercover

• metro, tube, subway system, subway

• resistance

• "belowground storage areas"; "underground caverns"

• "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"

• "the organization was driven underground"

• "water flowing underground"

• "in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'"

• Underground vs belowground
• Underground vs clandestine
• Underground vs cloak-and-dagger
• Underground vs hole-and-corner
• Underground vs hugger-mugger
• Underground vs hush-hush
• Underground vs secret
• Underground vs surreptitious
• Underground vs undercover
• Underground vs metro
• Underground vs tube
• Underground vs subway system
• Underground vs subway
• Underground vs resistance