Definition of "Underground" :

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

"In Paris the subway system is called the metro' and in London it is called the tube' or the underground'."

noun: a secret group organized to overthrow a government or occupation force

adjective: conducted with or marked by hidden aims or methods

"Underground resistance."

adjective: under the level of the ground

"Underground caverns."

adverb: beneath the surface of the earth

"Water flowing underground."

adverb: in or into hiding or secret operation

"The organization was driven underground."