Salt (sodium chloride) is different from other typical mining products, as it is necessary for living, it is an important part of our food. Thus salt is probably the first thing which was mined and it has been a sort of currency for millennia. The first known salt works are about 8,000 years old, found in the Shanxi province of China. Salt lakes and sea water were the source for salt water which was boiled in clay pots. In some areas of the world, the sun is sufficient to dry the water, at least during some parts of the year. In other areas salt exists on the surface and is mined in shallow open pits. In the Sahara the salt is mined and transported into the south, the subtropic prairies and rain forests with caravans.
Very early true salt mining, even underground, started. In areas without salt pans and far from the sea, it was vital to find salt deposits which could be mined, otherwise salt had to be transported over enormous distances. In the Austrian salt mines around Salzburg (mention the name), Celtic remains like shoes and wooden tools were found, which are some 3,000 years old. The mining in salt mines is a little easier than in other mines, as salt is rather soft, so it was possible even with stone age tools.
Today there are two different ways of underground salt mining. The first is mechanical mining by creating huge caverns and tunnels inside the salt, big enough to drive trucks inside. Salt is scratched off the walls by huge machines and transported to the surface in its solid form, on conveyor belts or heavy trucks. This works very well if the salt is either found in massive layers or in huge diapirs. And of course if the salt is very pure. The salt is used without any cleaning process for industrial processes and as road salt. The mined salt is actually unrefined salt, which means it contains various different salts beneath sodium chloride. It is also called rock salt or halite.
The second method is solutional mining. The salt is mined in the form of brine and the water then evaporated. Sometimes natural brine springs are used, but mostly fresh water is pumped into salt bearing layers and pumped back to the surface after it is saturated with salt. The brine has the maximum amount of dissolved salt which is possible, 26.4% salt by weight at a temperature of 15 °C. To clean the salt it is always important to dissolve it in water and filter it, which removes the insoluble solids like clay. The treatment with chemicals follows, which react with and precipitate most impurities. By evaporation in multiple stages, the different kinds of salts, like sodium, magnesium and calcium salts, are separated, a process called refining. And finally, if the salt is used for food an anticaking agent may be added.
Evaporation is the way the salt deposits all over the world were formed. Salt, contained in small amounts in any rock, is washed into the sea. In the sea the amount of salt rises since million of years. But when parts of the sea are cut off and the weather is dry, the water evaporates and the salt remains. The salt is deposited in a sequence, first the salt types which are hard to dissolve are precipitated, then the next and so on. The typical sequence of those evaporites is limestone, gypsum, sodium salt and finally potash. If the salt is then covered by mud or sand, the formation of the salt deposit is complete.
Other important geologic processes connected with salt are the halokinetic processes. Salt under high pressure and high temperature starts to flow very slowly. Although it looks still hard as a rock, it is deformed by the pressure of the rock above. If there are fractures in the rocks the salt starts to flow upwards forming huge diapirs or salt domes. Often those diapirs are mined, as they are less deep. Sometimes they reach the surface, even if the salt layer is three kilometers deeper, for example in northern Germany.
Salt mines are of impressive size, and as salt is hygroscopic, they are extremely dry too. This is a big difference to other mines. The air is very pure and full of salt, and many people think this air has healing powers. There are abandoned mines which are now used for halotherapy, especially in eastern Europe.
Some countries use abandoned mines for the storage of various goods. Because of the size, the chambers are easily accessible by trucks, so it is possible to store a lot of things, like documents, valuables, art, and food. During World War II salt mines were used to store art, because the dry air was important for their protection. Some are used to dump dangerous chemicals or (unfortunately) radioactive waste. Some mines are used for underground bicycle races.
Beneath mines producing sodium chloride, which is the salt we know from the kitchen, there are also mines producing potash (potassium carbonate). Looking similar to halite, except probably for the fact that it is reddish, this salt has a different chemical structure based on potassium instead of sodium. It is used mostly as fertilizer, but also in the manufacture of glass and soap.