Kitted out with hard hats and headlamps, our rucksacks filled with flasks and sandwiches, we head up the Nent Valley between barren mounds of spoil. An arched opening burrows into the hillside, the dark adit (passage) of Smallcleugh mine. Any fears I have about travelling deep underground evaporate in the shared excitement of our party led by three guides. We’re immediately plunged into a different world. Lights flicker over the tunnel’s sandstone blocks, wellies splosh through water and the smells are of damp stone and earth. Delicate white stalactites hang from the roof, their tips glistening with droplets. As the walls become rock-hewn, they are rust-coloured, with pockets of minerals gleaming in hollows. Begun in 1770, Smallcleugh produced an immense amount of lead ore and zinc before it closed in the 1920s. Many miles of passages thread through different levels and it would be easy to get lost. Along the main route run parallel rails for horse wagons. In the walls, 150-year-old wooden chutes remain, down which material from upper levels would be channelled into trucks. The miners’ knowledge of geology was extensive, their stonework beautiful in its craft and detail. They worked along seams of lead or hollowed out large areas of ore called “flats”, chambers where we pause and take it all in. We turn off our headlamps to experience total dark, that utter blackness that is 300ft below ground. It is then a five-minute slither and crawl down a crosscut called Hetherington’s Misery. My hat bumps against the roof and I’m glad I have padded gloves and knee pads. The lure is the Ballroom, a cavernous space or “stope”, where the local Masonic order held a dinner party in 1901. I’m in awe of the vast amount of rock that was extracted just using picks, and am moved to still see soot marks from the miners’ candles. The Ballroom is dramatic, but it’s in a narrow tunnel that I discover magic. Between sides of grey rock, the sandstone roof curves away, glimmering with thousands of water droplets, sparkling like nets of Christmas lights. Known as the Upside Down River, it’s a dazzling phenomenon deep in the earth.