underground

Definitions of underground
1

adv beneath the surface of the earth

“water flowing underground

adv in or into hiding or secret operation

“the organization was driven underground

adj under the level of the ground

underground caverns”
Synonyms:
belowground
subsurface
beneath the surface

adj conducted with or marked by hidden aims or methods

underground resistance”
Synonyms:
clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, undercover
covert
secret or hidden; not openly practiced or engaged in or shown or avowed

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

Synonyms:
metro, subway, subway system, tube
Type of:
railroad, railroad line, railway, railway line, railway system
line that is the commercial organization responsible for operating a system of transportation for trains that pull passengers or freight

n a secret group organized to overthrow a government or occupation force

Synonyms:
resistance
Types:
Maquis
the French underground that fought against the German occupation in World War II
Type of:
revolutionary group
a political unit organized to promote revolution

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