A salt mine is an operation that extracts rock salt from massive underground deposits left behind by ancient seas and lakes. These deposits, some dating back more than 500 million years, formed when bodies of saltwater evaporated faster than they were replenished, leaving thick layers of crystallized salt buried under accumulated rock and sediment. Today, roughly 270 million metric tons of salt are mined worldwide each year, feeding demand for everything from road de-icing to food production to chemical manufacturing.
How Salt Deposits Form
Salt deposits begin with shallow, enclosed bodies of water. When evaporation outpaces the inflow of fresh seawater, the water grows increasingly salty until dissolved minerals start crystallizing out. The lightest minerals settle first, followed by heavier salts. For a permanent deposit to form, the salt laid down during dry seasons has to survive without dissolving away during wet ones. When that balance holds over thousands of years, thick beds of rock salt, sometimes hundreds of feet deep, accumulate on the basin floor.
Over geologic time, these salt beds get buried under layers of sediment that compress into rock. The oldest known rock salt deposits date to the Cambrian period, roughly 540 million years ago. Many of the deposits mined today in North America and Europe formed during the Permian period, around 250 to 300 million years ago, when vast shallow seas covered large stretches of what are now landlocked continents. The salt sits in flat, tabular layers that can extend for miles, making them ideal targets for mining.
Two Main Ways Salt Is Mined
Salt mines fall into two broad categories: conventional underground mines and solution mines. The method depends on the depth, thickness, and accessibility of the deposit.
Room-and-Pillar Mining
Most underground salt mines use a technique called room-and-pillar mining. Heavy machines called continuous miners cut directly into the salt face, carving out large rectangular chambers (the “rooms”) while leaving intact columns of salt (the “pillars”) standing at regular intervals to support the rock above. A separate machine called a roof bolter drives long steel bolts into the ceiling to reinforce it. The process repeats in a grid pattern, with the miner advancing from one face to the next while bolting crews work behind it.
This method works at a wide range of depths, from as shallow as 60 feet to more than 2,500 feet below the surface. What it requires is a flat, uniformly thick salt bed with minimal tilt, no more than about 15 degrees of slope. The salt itself needs to be structurally strong enough to hold up the roof once rooms are carved out. The result, over decades of mining, is a vast underground network of tunnels and open chambers that can stretch across thousands of acres.
Solution Mining
When a salt deposit is too deep or too thin to mine conventionally, operators use solution mining instead. Clean water is pumped down a well and injected into the salt formation, where it dissolves the rock salt into brine. That saturated brine is then pumped back to the surface through a separate channel in the same well. At processing plants on the surface, the water is evaporated off, typically using vacuum evaporators, leaving behind pure crystallized salt. The U.S. Environmental Protection Agency regulates these injection wells because of the potential for groundwater contamination.
Scale of the Global Salt Industry
China is the world’s largest salt producer, turning out an estimated 53 million metric tons in 2023. The United States ranks second at about 42 million metric tons, followed by India at 30 million, Germany at 15 million, and Australia at 14 million. In the U.S., seven states produce about 95% of all domestic salt: Kansas, Louisiana, Michigan, New York, Ohio, Texas, and Utah.
The world’s largest underground salt mine sits 1,800 feet beneath Lake Huron near Goderich, Ontario. Operated by Compass Minerals since 1990, the Goderich mine has been in continuous production since 1959. Its depth is roughly equal to the height of Toronto’s CN Tower. In the U.S., the Detroit Salt Company runs a 1,500-acre subterranean complex about 1,100 feet below the city of Detroit.
Why Salt Was Once Called “White Gold”
Before refrigeration existed, salt was the only reliable way to keep meat and fish from spoiling. Navies and merchant fleets depended on salt pork, salt beef, and salt fish to feed crews on long voyages. The Newfoundland cod fishery alone consumed 25,000 tons of salt a year. By 1772, Britain’s North American colonies were importing nearly 40 million pounds of salt annually from the Caribbean. Entire economies were built around salt production. The Turks and Caicos Islands survived almost entirely on salt profits from 1678 to 1964, and salt taxes once made up a full quarter of the Bahamian government’s revenue.
Early methods were labor-intensive. When Roman forces arrived in Britain in the first century, they found local tribes making salt by pouring brine onto red-hot charcoal and scraping off the crystals that formed as the water boiled away. In the Caribbean, workers raked salt from shallow evaporation pans by hand. One archaeological experiment showed that 16 men using conch shells as scoops could gather about 7,840 pounds of salt in six hours from a natural salt pan.
What Happens When Salt Mines Fail
Salt has a property that makes old mines vulnerable: it slowly deforms and “creeps” into mined-out openings over timescales ranging from years to centuries. This gradual movement can destabilize the rock layers above, causing them to sag and eventually collapse. When that creates a pathway for water, the problem accelerates quickly. Salt dissolves easily, so any trickle of groundwater flowing into a mine widens its own channel, growing from a trickle to a cascade.
The consequences can be dramatic. The Retsof Salt Mine in New York, one of the largest in the Western Hemisphere, experienced a roof collapse that led to catastrophic flooding. As the mine filled with water, it drained the aquifers above it, contaminating the local groundwater with brine and rendering domestic wells unusable. Natural gas trapped in surrounding rock migrated through water wells and entered homes. Sinkholes formed at the surface. Some mines leak slowly for decades before a section of rock suddenly gives way. Others are destroyed when a misplaced drill hole punctures the mine ceiling from above.
Salt Mines as Storage Vaults
The same geological stability that keeps salt deposits intact for hundreds of millions of years makes abandoned salt mines attractive for long-term storage. Salt rock is impermeable to liquids and gases, and the creeping behavior that threatens active mines actually works as an advantage for storage: over time, the salt slowly seals around anything placed inside it, encapsulating it completely.
Governments and private companies use decommissioned salt mines to store sensitive records, film archives, and other materials that need stable temperature and humidity. More controversially, salt formations have been studied as repositories for radioactive waste. The overlying rock layers act as a barrier, and any radioactive material migrating upward through groundwater would be greatly diluted during its passage through hundreds of feet of rock. The groundwater surrounding salt formations already contains dissolved salt, which researchers factor into safety models for predicting how contaminants would move through the system over thousands of years.
Salt Caves and Respiratory Therapy
In Eastern Europe, natural salt caves have been used for generations to relieve symptoms of respiratory illness. The modern version, called halotherapy, recreates this environment in above-ground chambers where finely ground salt particles are dispersed into the air. The idea is that inhaling microscopic salt crystals helps loosen mucus in the airways, making it easier to cough up and improving airflow.
Some clinical observations have been encouraging. Patients with chronic obstructive pulmonary disease who underwent speleotherapy (therapy in actual salt caves) showed improvements in immune function and reduced inflammation during flare-ups in one study, with 97.8% of the treatment group showing improved immune markers compared to 67.5% in a control group. Other research has reported decreased airway obstruction, less frequent coughing, better sleep, reduced fatigue, and lower medication use after halotherapy sessions. The therapy has also been associated with relief from skin conditions like eczema. Still, the overall body of evidence remains limited, and halotherapy is generally considered a complementary approach rather than a standalone treatment.