Adjective
1. belowground
2. subsurface, clandestine
3. cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, on the quiet, secret, surreptitious, undercover, covert, guerrilla
Noun
1. guerilla, irregular, partisan, partizan, resistance
2. revolutionary group, metro
2. subway, tube, railway, railroad, railroad line, railway line, railway system