It’s a doorway to darkness, an opening cut into the side of a hill. It leads to a long, low tunnel. I have to crouch down to avoid hitting my head. All we can hear is the crunch of our feet on the ground. All we can see are the spots of light from our torches. We walk down into the darkness. My guide, Andrew Watson, points his torch at a hole in the tunnel wall. It’s about 2ft high. Andrew used to be a coal miner and tells me the 2ft-high tunnel is typical of the ones he used to have to crawl down when he worked in the mines. I tell him it sounds hellish. He shrugs. You get used to it, he says.