underground

Other forms: undergrounds

Definitions of underground

1. adverb

   beneath the surface of the earth
   “water flowing underground”
2. adverb

   in or into hiding or secret operation
   “the organization was driven underground”
3. adjective

   under the level of the ground
   “underground caverns”

   synonyms: belowground

   subsurface

   beneath the surface
4. adjective

   conducted with or marked by hidden aims or methods
   “underground resistance”

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

   covert

   secret or hidden; not openly practiced or engaged in or shown or avowed
5. noun

   an electric railway operating below the surface of the ground (usually in a city)
   synonyms: metro, subway, subway system, tube

   type of:

   railroad, railroad line, railway, railway line, railway system

   line that is the commercial organization responsible for operating a system of transportation for trains that pull passengers or freight
6. noun

   a secret group organized to overthrow a government or occupation force
   synonyms: resistance

   types:

   Maquis

   the French underground that fought against the German occupation in World War II

   type of:

   revolutionary group

   a political unit organized to promote revolution