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'" ;

1. (

• 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" ;

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" ;