Dubiously claimed as ancient (8000 years old and used by the druids... apparently), this is a 20 mile long complex of 12th – 17th century mineworkings & caves, located under chislehurst.
Watch out for the opening hours:
During summer holidays, open 7 days a week. Other times open Wednesday to Sunday, ie closed Monday and Tuesday (grrr a wasted morning grrr). Tours every hour 10 till 4.