underground
un-der-ground
adjective
Located or operating beneath the Earth's surface; hidden from view.
"We explored an underground cave system."
noun
A railway system running in tunnels below a city, especially London's Tube.
"I caught the Underground to Piccadilly Circus."
adjective
Operating secretly, often in opposition to a government or authority.
"They joined an underground resistance movement."
Thesaurus
Synonyms: subsurface, belowground, surreptitious, hugger-mugger, secret, clandestine, hush-hush, cloak-and-dagger, undercover, covert, hole-and-corner, resistance