Tin

They've been mining tin, copper and other metals down in Cornwall for thousands of years. Some of the mines extend out very deep under the sea We caught the bus to Geevor tin mine which only closed in 1990 due to a fall in the price of tin. However tin is still a much sought after component in all our devices so you never know.....
We walked along the coastal path back towards St Just where we continued home by bus. 
This whole area has such an  interesting industrial history, worth further exploration.
LooseCanon has blipped a lovely mono photo of part of Botallack mine and this is a separate building a bit farther on. I'm searching for the name!
Extra shows the red colouring of the soil around the Geevor site.