If you're interested in salt mines, and also in that general part of the world, take a look at Wieliczka and Bochnia[0][1], operating as commercial salt mines for 700 years (until the 1990s). Even the chandeliers are made of salt crystal. There's also a health resort for respiratory ailments 135m underground.
[0] https://whc.unesco.org/en/list/32/
[1] https://visitworldheritage.com/en/eu/royal-salt-mines-in-wie...