When I was a kid living in Hollywood, I was shown an underground club that had folk live music, poetry readings, and occasionally French porn movies and served cider and other beverages that I was too young to investigate. It was called the "Fifth Estate" and it was under Hollywood boulevard and you had to know where it was to find the entrance - there weren't any signs!
It was actually underground.
That’s how these type places got their name, usually in dark, unused storage places from previous centuries.
Prohibition era ‘Speakeasys’, French Anti-Nazi resistance fighters, Alternate Rock music clubs of the 60’s and 70’s, Actual band name Velvet Underground...........