underground - noun

a secret group organized to overthrow a government or occupation force

an electric railway operating below the surface of the ground (usually in a city)

underground - adjective

• under the level of the ground
• conducted with or marked by hidden aims or methods

underground - adverb

• in or into hiding or secret operation
• beneath the surface of the earth

underground - thesaurus

belowground   clandestine   metro   resistance   secret   subway   surreptitious   tube   undercover