, in
Slănic, Prahova, Romania. The railing (lower middle) gives the viewer an idea of scale. Before the advent of the modern
internal combustion engine and earth-moving equipment, mining salt was one of the most expensive and dangerous of operations because of rapid
dehydration caused by constant contact with the salt (both in the mine passages and scattered in the air as salt dust) and of other problems caused by accidental excessive sodium intake. Salt is now plentiful, but until the
Industrial Revolution, it was difficult to come by, and salt was often mined by
slaves or
prisoners. Life expectancy for the miners was low. The earliest found salt mine was in
Hallstatt, Austria where salt was mined, starting in 5000BC. As salt is a necessity of life, pre-industrial governments were usually keen to exercise stringent control over its production, often through direct ownership of the mines. Whereas the collection of most
taxes generally required at least the grudging cooperation of the upper classes, ownership of salt mines could provide monarchs with a lucrative source of income for which they did not need to rely on the goodwill of other strata of society such as the
nobility to remit to the monarch. For example,
Polish king
Casimir the Great relied on salt mines for over a third of his revenue in the
14th century. Ancient China was among the earliest civilizations in the world with
cultivation and trade in mined salt. They first discovered
natural gas when they excavated rock salt. The Chinese writer, poet, and politician
Zhang Hua of the
Jin dynasty wrote in his book
Bowuzhi how people in
Zigong,
Sichuan, excavated natural gas and used it to boil a rock salt solution. The ancient Chinese gradually mastered and advanced the techniques of producing salt. Salt mining was an arduous task for them, as they faced geographical and technological constraints. Salt was extracted mainly from the sea, and salt works in the coastal areas in late imperial China equated to more than 80 percent of national production. The Chinese made use of
natural crystallization of salt lakes and constructed some
artificial evaporation basins close to shore. In 1041, during the
Song dynasty, a well with a diameter about the size of a bowl and several dozen feet deep was drilled for
salt production. In Southwestern China, natural salt deposits were mined with
bores that could reach to a depth of more than , but the yields of salt were relatively low. Salt mining played a pivotal role as one of the most important sources of the Imperial Chinese government's revenue and state development. Most modern salt mines are privately operated or operated by large multinational companies such as
K+S,
AkzoNobel,
Cargill, and
Compass Minerals. == Mining regions around the world ==