Wiktionary
undergrounder
n. 1 An underground publication or movie. 2 One who is part of a secret or underground society or subculture.
Usage examples of "undergrounder".
Something, as it happened, turned up very soon, for as Speedy and Marygolden entered the public square of Subterranea, a crowd of undergrounders, catching sight of them, forgot to whisper and burst into earsplitting shrieks and yells.
From their expressions, none of the students had actually met any of the undergrounders before.
Marygolden, more interested than alarmed, pressed her pretty face against the window glass, but Speedy, as the undergrounders with bars and clubs fell upon the torpedo, began to feel terribly anxious.