The Wielickza Salt Mine near Krakow, Poland

The Wieliczka Salt Mine reaches a depth of 327 metres and extends via horizontal passages and chambers for over 287 km. The rock salt is naturally of varying shades of grey, resembling unpolished granite rather than the white crystalline substance that might be expected.
Since the 13th century, brine welling up to the surface had been collected and processed for its sodium chloride (table-salt) content. In this period, wells began to be sunk, and the first shafts to be dug to extract the rock salt. In the late 13th to the early 14th century, the Saltworks Castle was built.
Source: Wikipedia