Subsurface mining takes place wherever valuable mineral or fuel deposits sit too deep beneath the surface for open-pit extraction to be practical. That means locations on every inhabited continent, from coal seams under the Appalachian Mountains to gold reefs nearly 4 kilometers below South Africa’s surface. The specific sites are determined by geology, economics, and the type of resource being extracted.
Why Some Deposits Require Underground Mining
The basic decision between surface and underground mining comes down to how much rock sits on top of the ore. Engineers express this as a “stripping ratio,” the volume of waste material that must be removed for every unit of ore recovered. At active iron ore operations in Minnesota, for example, the stripping ratio runs about 1:1. But as mining moves toward deeper portions of the same iron formation, that ratio climbs to 4:1 or even 6:1 within roughly a mile. At those ratios, hauling away waste rock costs more than the ore is worth, and underground mining becomes the only viable option.
Depth compounds the problem quickly. In Minnesota’s Mesabi Range, the iron-bearing rock layer dips southward at 5 to 10 degrees. For every mile mining advances in that direction, the ore drops about 700 feet deeper. Once the target zone sits more than 1,000 feet below the surface, underground methods are typically the only realistic choice. Similar geometry plays out in coal basins, gold fields, and base-metal deposits worldwide.
Major Regions for Underground Coal Mining
Coal is the most widespread resource extracted underground, and the United States illustrates how geography shapes where it happens. The Appalachian coal region, spanning West Virginia, Pennsylvania, Eastern Kentucky, Virginia, Alabama, Ohio, Maryland, and Tennessee, is heavily dependent on underground methods. About 80% of the coal produced in Appalachia comes from underground mines, and those operations account for 58% of all underground coal mined in the country. West Virginia leads the region and ranks as the second-highest coal-producing state nationally.
The Interior coal region, covering Illinois, Indiana, Western Kentucky, and several surrounding states, also relies heavily on underground extraction. Underground mines supply 68% of the region’s coal output, with Illinois alone producing about 6% of total U.S. coal. These mines typically work relatively flat-lying seams using longwall or room-and-pillar methods suited to the geology of thick, horizontal coal beds.
Outside the U.S., major underground coal operations run in China (particularly Shanxi and Inner Mongolia provinces), Poland’s Silesian Basin, Australia’s Hunter Valley and Bowen Basin, and India’s Jharia coalfield. China operates the largest number of underground coal mines in the world by a wide margin.
South Africa’s Deep Gold Mines
The deepest subsurface mining on Earth happens in South Africa’s Witwatersrand Basin, a gold-bearing geological formation near Johannesburg. The Mponeng mine, operated by Harmony Gold, is the world’s deepest active mine, with extraction happening between 3,160 and 3,740 meters below the surface. That’s more than two miles down. The mine is currently undergoing a life extension project approved in early 2024, with annual production guidance of roughly 280,000 to 311,000 ounces of gold.
Mponeng is far from alone. South Africa dominates the list of the world’s deepest mines. TauTona reaches 3.9 kilometers, Driefontein extends to 3.4 kilometers, and Kusasalethu, Kloof, Moab Khotsong, South Deep, and several others all exceed 2.3 kilometers. Many of these mines also recover uranium as a byproduct. The concentration of ultra-deep mining here reflects the unique geology of the Witwatersrand Basin, where narrow gold reefs extend to extraordinary depths in relatively stable rock.
Other Notable Underground Mining Locations
Canada hosts some of the deepest non-gold underground mines. The LaRonde mine in Quebec reaches 3.26 kilometers, extracting gold, copper, silver, and zinc. The Kidd Mine in Ontario, one of the world’s deepest base-metal operations, extends past 3 kilometers for copper and zinc. The Creighton Mine near Sudbury, Ontario, reaches 2.39 kilometers for nickel. These Canadian operations reflect the mineral-rich geology of the Canadian Shield, one of the oldest and most stable rock formations on the planet.
In the United States, the Lucky Friday Mine in Idaho’s Silver Valley reaches 2.9 kilometers, making it one of the deepest mines in the Western Hemisphere. It produces silver, lead, and zinc. Underground mining also occurs across Nevada (gold), Missouri (lead), and New Mexico (potash), though at shallower depths.
Brazil’s Morro Velho gold mine reaches 3 kilometers. Salt mining takes place underground in locations ranging from Louisiana to Pakistan to Poland, where the Wieliczka Salt Mine has operated since the 13th century. Potash, limestone, and various industrial minerals are extracted underground across Europe, Russia, and South America.
How Mining Methods Match the Geology
The specific technique used underground depends on the shape, angle, and strength of the deposit. Room-and-pillar mining works best for horizontal or near-horizontal beds in reasonably strong rock, leaving pillars of unmined material to support the ceiling. This is the standard approach for coal, salt, limestone, and lead in flat-lying formations.
Longwall mining is another method applied to thin, horizontal deposits, primarily coal. A mechanical shearer moves back and forth across a long face of rock while hydraulic supports hold up the roof immediately above the work area. As the shearer advances, the roof behind it is allowed to collapse in a controlled manner. This method is common in U.S. Appalachian and Interior basin coal mines as well as in Australian and Chinese operations.
Steeply dipping ore bodies call for different approaches. Shrinkage stoping and open stoping both work in strong, competent rock where the deposit angles sharply downward, suitable for iron and base metals. Cut-and-fill mining, where each mined-out layer is backfilled with waste material before the next layer is extracted, is preferred for gold deposits in weaker or irregular rock. It allows for selective mining and adapts well to unpredictable ore body shapes, which is why it’s used in many of the deep South African gold operations.
For massive, weaker ore bodies, caving methods bring down large volumes of rock at once. Block caving undercuts the ore and lets gravity do much of the work as material collapses into the void. Sublevel caving works in strong ore bodies surrounded by weak host rock, and it’s used for large, steeply dipping deposits where the hanging wall will fracture under controlled conditions.
Environmental Concerns at Underground Sites
Underground mining produces a smaller surface footprint than open-pit operations, but it creates its own set of environmental issues. Subsidence is the most visible. When material is removed from beneath the surface, the overlying rock can shift and sink. The severity depends on the mining method, the depth of extraction, the thickness of the deposit, local topography, and the strength of the rock mass above the void. Longwall mining, which deliberately allows the roof to cave, can produce noticeable surface subsidence directly above the mined area.
Acid mine drainage is another persistent problem. Many ore bodies contain sulfide minerals that, once exposed to air or oxygenated groundwater during mining, oxidize and produce acidic water loaded with sulfates, heavy metals, and metalloids. This contaminated water can seep into streams and aquifers long after a mine closes. It’s one of the most significant pollution sources associated with the mining industry globally, affecting former mining regions from Appalachia to Portugal to South Africa.
Deep mines also face challenges with heat (rock temperatures at 3+ kilometer depths can exceed 60°C), water management from intersecting aquifers, and the energy-intensive ventilation systems needed to keep air breathable thousands of meters underground. These factors make deep subsurface mining among the most technically demanding and expensive forms of resource extraction.