underground in WordNet English dictionary

1. under the level of the ground
   "belowground storage areas"; "underground caverns"
   belowground  underground 
   
2. 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"
   clandestine  cloak-and-dagger  hole-and-corner  hugger-mugger  hush-hush  secret  surreptitious  undercover  underground 
   

1. 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'"
   metro  tube  underground  subway system  subway 
   
2. a secret group organized to overthrow a government or occupation force
   underground  resistance 
   

1. beneath the surface of the earth
   "water flowing underground"
   underground  underground 
   
2. in or into hiding or secret operation
   "the organization was driven underground"
   underground  underground