underground
1. [ adjective ] under the level of the ground
Synonyms: belowground
Examples:

"belowground storage areas" "underground caverns"

Related terms: subsurface
2. [ noun ] (politics) a secret group organized to overthrow a government or occupation force
Synonyms: resistance
Related terms: revolutionary_group Maquis
3. [ adjective ] conducted with or marked by hidden aims or methods
Synonyms: secret undercover clandestine surreptitious hush-hush hugger-mugger on_the_quiet hole-and-corner cloak-and-dagger
Examples:

"clandestine intelligence operations" "cloak-and-dagger activities" "secret missions" "surreptitious mobilization of troops" "an undercover investigation" "underground resistance"

Related terms: covert
4. [ adjective ] (military) used of independent armed resistance forces
Synonyms: guerrilla irregular guerilla
Examples:

"guerrilla warfare" "partisan forces"

Related terms: partisan
5. [ adverb ] beneath the surface of the earth
Examples:

"water flowing underground"

6. [ adverb ] in or into hiding or secret operation
Examples:

"the organization was driven underground"

7. [ noun ] (transportation,business) electric underground railway
Synonyms: subway metro tube
Related terms: railway
Similar spelling:   undergrad
  undergrow
  undergrowth
  underhung
  Underwood
  underbody
  underdone
  undertone
  Underdown
  undergird
  undergraduate
  understood
  underslung
  undersign
  undershrub
  underhand
  undercharge
  underbodice
  underworld
  underspend
  understand
  underperform
  underhanded
  underpopulated
  underproduce
  underfur
  undertow
  underdog
  undercoated
  underling
  underwing
  underboss
  underwear
  underclothed
  underfoot
  underpart
  undersoil
  undershot
  underpin
  undercoat
  under_control