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, due to rapid dehydration caused by constant contact with the salt (both in the mine passages and scattered in the air as salt dust), among other problems borne of accidental excessive sodium intake. While salt is now plentiful, until the Industrial Revolution it was difficult to come by, and salt mining was often done by slave or prison labor and life expectancy among those sentenced was low.
In ancient Rome, salt on the table was a mark of a rich patron; those who sat nearer the host were “above the salt”, and those less favored were “below the salt”. The Roman historian Pliny the Elder stated as an aside in his Natural History’s discussion of sea water, that “[I]n Rome … the soldier’s pay was originally salt and the word ‘salary’ derives from it …”
Ancient China was among the earliest civilizations in the world with cultivation and trade of salt mining. 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 the ancient Chinese, who faced geographical and technological constraints. Salt was mainly extracted from the sea, and salt works in the coastal areas in late imperial China equated to more than 80 percent of national production. In conjunction with this, the Chinese made use of natural crystallization of salt lakes and constructed some artificial evaporation basins close to shore.In 1041, during the medieval 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 1000 meters but the yields of ground and salt were relatively low. As salt was a necessity of life for human civilization, salt mining played a pivotal role as one of the most important sources of Imperial Chinese government revenue and state development