1. (n.) The place or space beneath the surface of the ground; subterranean space.

2. (a.) Being below the surface of the ground; as, an underground story or apartment.

3. (a.) Done or occurring out of sight; secret.

4. (adv.) Beneath the surface of the earth.

Amtrak Charley L Trojan horse VC Vietcong abstruse agreement to disagree alienated alienation alternative antiestablishment apostasy at odds with at rest at variance with avant-garde back door back stairs back way back-door backstairs baggage train beclouded behind the curtain behind the scenes behind the veil blind bolt-hole branch breakaway buried bushfighter bushwhacker cable railroad cable railway casual choo-choo clandestine close clouded cog railroad cog railway collaborationist collaborator concealed counter-culture covered covert covert way covertly crypto deep-buried difference differing disaccord disagreeing disagreement disapprobation disapproval disparity dissatisfaction dissension dissent dissentience dissentient dissenting dissidence dissident diversity dropping out eclipsed el electric electric railway electric train elevated elevated railway embankment escalier derobe escape hatch escape route experimental express express train extremists feeder feeder line feline fellow traveler fifth column fifth columnist flier fraternizer freight freight train freighter funicular furtive goods train gravity-operated railway guerillas guerrilla hid hidden hidlings hole-and-corner horse railway hugger-mugger in a cloud in a corner in a fog in a whisper in darkness in eclipse in hidlings in opposition in purdah in secret in the background in the dark in the wings incommunicado insurgents insurrectionists interurban irregular junction latent light railroad lightning express limited line local main line maquis maquisard metro milk train minority opinion monorail mysterious nobody the wiser nonagreement nonassent nonconcurrence nonconforming nonconformist nonconformity nonconsent obfuscated obscure obscured occult opposing opposition parliamentary parliamentary train partisan passenger train private privy quiet rack railway rack-and-pinion railroad rack-and-pinion railway radical rail rail line railroad railroad train railway rattler recondite recusance recusancy recusant rejection repudiation resistance resistance fighter resting in peace revolutionary roadbed roadway rolling stock saboteur secession secluded secluse secret secret exit secret passage secret staircase secretly sectarian sectary security risk sequestered shifty shuttle shuttle train side door sidetrack siding skulking slinking slinky sly sneaking sneaky sotto voce special stealthy streamliner street railway streetcar line sub rosa subterranean subterraneous subversive subversives subway sunken surreptitious switchback terminal terminus track train tram tramline trestle trolley line trunk trunk line tube turnout under an eclipse under cover under house arrest under the breath under the rose under wraps under-the-counter under-the-table undercover underfoot underground fighter underground railroad underground route underhand underhanded unknown