The primary definition of underground means hidden, but most of the secondary definitions apply here. adj. 3a: existing outside the establishment, as in "an un

The primary definition of underground means hidden, but most of the secondary definitions apply here. adj. 3a: existing outside the establishment, as in "an underground literary reputation" n. 3c: an unofficial, unsanctioned, or illegal but informal movement or group The journalist might even have intended: n. 3b: a clandestine conspiratorial organization set up for revolutionary or other disruptive purposes especially against a civil order