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a city's railway system that is below the ground, usually in big cities
metro
subway system
tube
a secret group organized to overthrow a government or occupation force
resistance
situated, operating, or existing beneath the surface of the ground
belowground
conducted with or marked by hidden aims or methods
clandestine
cloak-and-dagger
hole-and-corner(a)
hugger-mugger
hush-hush
under the surface of the earth
in or into hiding or secret operation
