A salt mine is an excavated area on or beneath the earth's surface created for the purpose of extracting this sought-after mineral. This widely used crystalline mineral varies in color, and may be found in seawater or on land. It is necessary to mine salt to get to underground deposits. A salt mine has shafts for entry and exit and is typically comprised of rooms created in a checkerboard pattern. Extraction and processing includes blasting the crystals free, then crushing them several times before sending them to the surface for further processing.
Also referred to as sodium chloride, salt is a mineral with a cubic, crystalline formation. Its color ranges from grayish to transparent or frosty white to pink, depending on its purity and mineral composition of the parent rock. It is widely used for industrial applications as well as in food.
Salt is a very abundant mineral, and is most commonly found in seawater, making up 77 percent of dissolved solids there. Deposits on the earth's surface are a result of the past evaporation of bodies of water. Salt deposits can also be found underground, in domes or veins among layers of sedimentary rock. In the latter cases, extraction requires the creation of a salt mine.
Like coal operations, a salt mine uses shafts to allow personnel, supplies, and mined rock to be transported below and above ground. One shaft is designated for people, and another for mined salt and supplies that are pulled up and lowered down carefully. Shafts also provide fresh air to working miners. Mining rooms are usually created in a checkerboard pattern so that a portion of the salt, about 35 to 55 percent, is left in the form of pillars. These offer structural support for the mine.
The mining process is labor-intensive and begins with cutting a horizontal slot beneath a salt wall, into which blasted pieces will eventually fall. Holes about 10 feet (about 3 m) or deeper are drilled into the side, and dynamite is placed inside them. The explosives are set off remotely when no personnel are in the salt mine, and the rock breaks and falls onto the floor. Miners gather the mineral, and it travels along a conveyor belt where it's crushed several times before being sent to the surface. The salt is then sieved through several screens and stored for packaging and shipping.
Until the industrial revolution, salt was scarce, and mining it was considered extremely dangerous and costly. Before this time, salt extraction was carried out primarily by slaves who were, in fact, traded for the commodity in places like Ancient Greece. The modern salt mine was developed after the creation of the internal combustion engine.