noun a secret group organized to overthrow a government or occupation force.
— synonyms: resistance
noun an electric railway operating below the surface of the ground (usually in a city).
— synonyms: metro, tube, subway_system, subway
adjective under the level of the ground.
— synonyms: belowground
adjective conducted with or marked by hidden aims or methods.
— synonyms: clandestine, cloak-and-dagger, hole-and-corner(a), hugger-mugger, hush-hush, secret, surreptitious, undercover
Examples
“the French underground during World War II”
“There is an underground tunnel that takes you across the river.”
“These criminals operate through an underground network.”