You have it. There's a "scene" not really different from the SF scene that existed since the beat days (at least). The Valley is more square; this is more SF, some Marin, some down the coast to La Honda/Pescadero.

The person writing it up is just trying to make it seem a bit more exotic. "Underground"? My friends invite me to parties. People who aren't my friends don't. Does this make one of the set of parties "underground"?