Underground in this context means ‘Not licensed’, illegal, Secret known to only a few, not physically under the ground......
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.