This 'flue' tunnel is one of several in the underground network at Yarnbury Mine in the Yorkshire Dales - part of the finest lead mining remains in England. The flues run from the base of the hill where the lead was smelted up to a central chimney, or 'cupola'. The purpose of these tunnels was to channel the hot gases from the smelting process, allowing them to condense and deposit the precious metal lead along the tunnel walls. The flues would then be blasted with a rush of water from a large reservoir at the top of the hill which would wash the metal deposits down to a settling pool where the lead could be collected. There's over a mile of these tunnels in the area and they are fascinating to explore.