BTW, around here in Finland we had an undersea iron mine at the island of Jussarö. It last operated in 1954-1967.
Location: https://goo.gl/maps/FoJeqJRUS6R2
It's probably not unique; are there similar mines elsewhere? (I.e. the mine shaft is on dry land, but mine tunnels reach to dig minerals from areas that are actually underneath the sea floor).