underground (n.)
- alienation
- alternative
- apostasy
- avant-garde
- backstairs
- blind
- bolt-hole
- branch
- breakaway
- bushwhacker
- choo-choo
- close
- collaborationist
- collaborator
- covert
- difference
- disagreement
- disapprobation
- disapproval
- disparity
- dissatisfaction
- dissension
- dissent
- dissidence
- dissident
- diversity
- el
- electric
- elevated
- embankment
- express
- feeder
- feline
- flier
- freight
- freighter
- funicular
- guerrilla
- hid
- hugger-mugger
- irregular
- junction
- limited
- line
- local
- maquis
- maquisard
- metro
- monorail
- nonconformist
- nonconformity
- occult
- opposition
- partisan
- private
- privy
- quiet
- radical
- rail
- railroad
- railway
- rattler
- recusancy
- recusant
- rejection
- repudiation
- resistance
- revolutionary
- roadbed
- roadway
- saboteur
- secession
- secret
- sectarian
- sectary
- shuttle
- sidetrack
- siding
- skulking
- sly
- special
- streamliner
- subversive
- subway
- terminal
- terminus
- track
- train
- tram
- tramline
- trestle
- trunk
- tube
- turnout
- unknown
- variance
- withdrawal
underground (adv.)
underground (adj.)
- abstruse
- alienated
- alternative
- avant-garde
- backstairs
- blind
- branch
- breakaway
- buried
- casual
- clandestine
- close
- clouded
- concealed
- covered
- covert
- dissentient
- dissenting
- dissident
- el
- electric
- elevated
- experimental
- express
- feline
- funicular
- furtive
- hidden
- hole-and-corner
- hugger-mugger
- incommunicado
- irregular
- latent
- limited
- line
- local
- mysterious
- nonconforming
- nonconformist
- obscure
- occult
- opposing
- parliamentary
- partisan
- private
- privy
- quiet
- radical
- rail
- recondite
- recusant
- revolutionary
- secluded
- secret
- sectarian
- sequestered
- shifty
- sly
- sneaking
- sneaky
- special
- stealthy
- subterranean
- subterraneous
- subversive
- sunken
- surreptitious
- terminal
- track
- under-the-counter
- under-the-table
- undercover
- underhand
- underhanded
- unknown
- unobtrusive
- variance
