WordNet sense 1 (a secret group organized to overthrow a government or occupation force):

resistance

WordNet sense 2 (an electric railway operating below the surface of the ground (usually in a city)):

metro, subway, tube, subway system

WordNet sense 3 (under the level of the ground):

belowground

WordNet sense 4 (beneath the surface):

subsurface

WordNet sense 5 (conducted with or marked by hidden aims or methods):

clandestine, cloak-and-dagger, hush-hush, secret, surreptitious, undercover, hole-and-corner, hugger-mugger

WordNet sense 6 (secret or hidden; not openly practiced or engaged in or shown or avowed):

covert