The sinkhole that developed over the chimney collapse in December 2013 (photo: The Independent).
A major, near-vertical mineral vein hosted by faulted Lower Carboniferous limestone is concealed beneath the Namurian mudstone forming the southern slope of Hucklow and Eyam Edges. It has been mined underground from numerous localities in several phases during a period of three centuries. The depth of working increased progressively as more options became available to drain water out of the mines. Prior to the 20th century, these operations were limited to the selective extraction of higher-grade zones of galena. Miners left the gangue minerals largely intact, albeit honeycombed with a network of narrow tunnels and stopes. The overlying mudstone was also penetrated by shafts and tunnels. The modern Milldam Mine recovers the unworked, in-situ vein minerals (mainly fluorite, barite and calcite) by open stoping of the vein across its full width. Mined-out voids are backfilled with waste rock to provide long-term ground stability. The legacy of historical underground mining and the particular local geology has created situations where unsupported, weak roof zones above open cavities are vulnerable to sudden collapse, enabling voids to migrate through the overlying mudstone to form sinkholes at the ground surface.
 John Hunter, ‘Old mines and new sinkholes along the Hucklow Edge vein, Derbyshire’, Mercian Geologist 2015 18 (4)