undergrounds
un-der-grounds
noun
Plural of 'underground', referring to subway or metro systems in various cities.
"Many major cities have extensive undergrounds for public transport."
noun
Plural of 'underground', referring to secret political or resistance movements or networks.
"Several resistance undergrounds operated during the war."
Thesaurus
Synonyms: subsurface, belowground, surreptitious, hugger-mugger, secret, clandestine, hush-hush, cloak-and-dagger, undercover, covert, hole-and-corner, resistance