OK, I get the point about being subterranean, but what about the context of a subculture? Things like alternative,clandestine,secret(weak),unknown, revolutionary,unpopular, or whatever else might fit. The whole idea of not being being part of the mainstream culture...
It would be pointless to label what is and isn't underground, because, part of the nature of our consumer society, what once was underground will eventually be coopted into the mainstream, tamed down, and marketed away into insignificance, though not without having whatever life it had sucked out of it.