A circa-1930s postcard, via the always excellent blog of musician-artist Steve Roden:
Roden explains that the card shows “engineers testing radio reception from within the depths of ‘endless caverns’, new market, VA. the men in the image are listening to the radio, nearly a mile from the entrance to the caverns. the back states clearly that ‘musical reception’ was ‘exceptionally good’.” More at the original post, inbetweennoise.blogspot.com.