Blackheath in London rises above nearby Greenwich and has chalk geology. Over the years this has been exploited for building materials (both as stone and for burning in lime kilns) and for agriculture.. The resulting mines and deneholes have occasionally opened up over the years to the surprise of local residents.
The underground complex known as Jack Cade’s or Blackheath Cavern has been lost and re-found several times over the years. Between 1780 and 1853 it became a tourist attraction and operated as a show cave. It then disappeared off the radar until World War II when an entrance was re-excavated and it was evaluated as a possible air-raid shelter. The Borough Surveyor felt it to be too unstable and probably too small to pursue. No access now remains but hopefully one day its secrets will once again be open to explore.