underground

Adjective

• 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
    

• under the level of the ground
  • belowground storage areas
    
  • underground caverns
    

Adverb

• beneath the surface of the earth
  • water flowing underground
    

• in or into hiding or secret operation
  • the organization was driven underground
    

Noun

• 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'
    

• a secret group organized to overthrow a government or occupation force

Related words

• Synonyms: 
  clandestine , cloak-and-dagger , hole-and-corner(a) , hugger-mugger , hush-hush , secret , surreptitious , undercover , belowground , metro , tube , subway system , subway , resistance