Geevor Tin Mine, Cornwall

Until 1990 this was a working tin mine and major local employer, but now it is preserved as a monument to the Cornish tin mining industry. Some of the machinery was removed for scrap, but much remains. It is now a major exhibit, with a museum and the surface machinery areas open to the public. The preserved site extends over 67 acres. The mineshafts are not open to the public, but within the site you will see a scary looking black hole which is the mouth of one of the vertical shafts. Look out for the model somewhere in the museum which reveals the enormous complexity of the system of shafts and galleries, which extended deep undergoround and under the sea.
There is a guided underground tour into Wheal Mexico, an 18th century mine. This is shallow and mostly horizontal, but authentically twisty and cramped and dank. My sister survived the surface tour but balked at going into Wheal Mexico.
Allow several hours to see everything.
In the surrounding landscape are many relics of earlier mining enterprises, in the form of gaunt engine houses and rubble-strewn ore processing floors.