A salt mine is an underground excavation where rock salt, the mineral halite, is extracted from ancient geological deposits. These deposits formed millions of years ago when shallow seas evaporated, leaving behind concentrated layers of salt that were eventually buried under sediment and rock. Today, salt mines supply raw material for everything from chemical manufacturing to winter road treatment, and some retired mines have found surprising second lives as storage vaults, tourist attractions, and even medical facilities.
How Salt Deposits Form
Rock salt begins as seawater. Millions of years ago, shallow inland seas covered large portions of what are now landlocked regions. As those seas slowly evaporated, the dissolved salt became increasingly concentrated until thick layers of halite crystallized on the sea floor. Over geological time, newer layers of sediment and rock buried the salt, compacting it and sealing it off from rainwater that would otherwise dissolve it. The result is massive underground beds of solid salt, sometimes hundreds of feet thick, that can persist for hundreds of millions of years.
These deposits exist on every continent. Some of the most well-known formations date to the Permian Period, roughly 250 to 300 million years ago, though salt beds from other eras are mined around the world as well.
How Salt Is Mined
There are two primary methods for getting salt out of the ground, and the choice depends largely on how deep the deposit sits and what form the salt needs to take.
Room-and-Pillar Mining
This is the method most people picture when they think of a salt mine. A vertical shaft is drilled through the overlying rock to reach the salt bed, which can sit anywhere from 500 to 1,000 feet below the surface. Once miners reach the deposit, they remove salt in a checkerboard pattern, carving out large square caverns while leaving equally large square pillars of salt in place to support the ceiling and the rock above it. Workers blast the salt into manageable chunks, crush it, and haul it to the surface through the shaft in large buckets.
The caverns left behind can be enormous. Because salt is naturally dry, stable, and resistant to humidity fluctuations, these underground rooms maintain remarkably consistent conditions for decades or even centuries after mining stops.
Solution Mining
For deeper or less accessible salt beds, operators use solution mining instead. Freshwater is pumped down a cased well into the salt deposit, where it dissolves the halite and creates brine, a heavily saturated saltwater solution. That brine is then pumped back to the surface and evaporated to recover the solid salt. This method doesn’t create the dramatic underground chambers of room-and-pillar mining, but it’s effective for reaching deposits that would be impractical to access with conventional shafts.
What Mined Salt Is Used For
Only a small fraction of the salt pulled from mines ends up on your dinner table. The vast majority goes to industrial applications that most people never think about.
The chemical industry is the single largest consumer, accounting for about 42% of total salt sales in the United States. Within that sector, roughly 90% of the salt used as chemical feedstock arrives in brine form. Manufacturers use it primarily to produce chlorine and caustic soda (sodium hydroxide), two essential building blocks for plastics, paper, textiles, water treatment chemicals, and thousands of other products.
Highway deicing is the next biggest use, consuming about 37% of total salt. The rock salt spread on roads in winter is often minimally processed, crushed and screened but not refined to food-grade purity. The remaining salt production goes to food processing, agriculture, water softening, and various other industrial processes.
Second Lives for Retired Salt Mines
The same qualities that make salt deposits geologically stable, dry, cool, and shielded from surface weather, also make emptied-out mines useful long after the last load of salt is hauled up. Several former salt mines around the world have been repurposed in creative ways.
Secure Storage
Underground Vaults & Storage, a facility near Hutchinson, Kansas, has operated since 1959 in caverns roughly 650 feet below the surface. About 50 acres of hollowed-out salt chambers store government records, private assets, film reels, and movie props. The constant underground environment protects fragile materials from temperature swings, humidity, and natural disasters. The facility has even explored using the space for data centers.
In England’s Winsford salt mine, a company called DeepStore holds archival collections including the fashion archive of Laura Ashley, with hand-painted wallpaper, clothing, and other items spanning seven decades. During World War II, Nazis used salt mines like Austria’s Altaussee to hide looted artwork and valuables, taking advantage of the same natural protection from bombs and weather.
Health and Tourism
Some salt mines have become destinations for visitors and patients alike. Speleotherapy, the practice of spending time in underground salt environments to relieve respiratory conditions, has a following in parts of Europe. In Belarus, the National Speleotherapy Clinic operates inside underground salt caverns, offering treatment for patients with respiratory ailments and allergic diseases. Romania’s Praid Salt Mine housed a medical center among its tourist amenities before closing in 2025.
Preserving Human Knowledge
In Hallstatt, Austria, one of the world’s oldest salt mining regions, an artist named Martin Kunze launched the Memory of Mankind project. The idea is to transfer digital records of modern civilization onto durable ceramic tablets and store them deep inside the salt mine, where they could theoretically survive for thousands of years, long after hard drives and servers have failed.
Why Salt Mines Stay Stable
Salt has a few properties that make it unusually well-suited for underground excavation. It’s relatively soft compared to other mined minerals, which makes it easier to cut and blast. It’s also nearly impermeable to moisture when compacted, so the walls and ceilings of salt chambers don’t absorb water from the air. Underground temperatures stay constant year-round, typically in the range of 60 to 70 degrees Fahrenheit depending on depth and location. Humidity inside an active or retired salt mine stays low because the salt itself absorbs any moisture that enters.
These conditions explain why documents stored 650 feet underground in Kansas can survive in better shape than the same paper would in a climate-controlled building on the surface. They also explain why salt mines, unlike many other types of mines, rarely face the groundwater flooding problems that plague coal or metal operations, as long as the overlying rock layers remain intact.