Subsurface Mining Examples: 8 Methods and Real Mines

Subsurface mining means taking ore from below ground without stripping away the rock above it. Common examples are room and pillar coal mines, longwall coal faces, stoped gold and base-metal mines, block caves such as Oyu Tolgoi in Mongolia, and wellfields that dissolve uranium in place. In the United States, 177 underground coal mines produced about 206 million short tons in 2024 (EIA, Annual Coal Report 2024).

This guide gives eight methods, a real operation for each, and the deposit shape that points to each one. It then covers the mapping techniques that decide which method is possible, and ends with a calculator for the ore grade an underground mine needs to break even. It is written for consultants and project owners who have to explain a method choice to a client or a board. For the plain definition and how it differs from pit mining, see our guide to subsurface and surface mining impacts.

Subsurface mining examples at a glance

The table pairs each method with a named operation. A stope is the opening left when ore is taken out. A sublevel is a working tunnel between the main haulage levels. Dip is the angle of the ore body from horizontal.

Eight methods and the mines that use them

1. Room and pillar: coal, potash and other flat beds

Miners cut a grid of rooms across a flat bed and leave columns of ore, called pillars, to hold up the roof. The method is used for coal, gypsum, limestone, talc, soda ash and potash, among other deposits. It is simple and easy to mechanize. The cost is lost ore, because whatever stays in the pillars is not sold.

In US coal, continuous and conventional methods together produced about 92.8 million short tons in 2024, roughly 45% of underground output (EIA). Saskatchewan-style potash mining is a close relative: Nutrien describes cutting ore with two- and four-rotor continuous boring machines, then moving it by conveyor to underground bins and a hoist.

2. Longwall: high-volume coal

A shearer machine travels along a long coal face, cutting a slice each pass. Hydraulic roof supports move forward behind it, and the roof collapses into the mined-out space. A commonly quoted comparison puts coal recovery near 80% for longwall against about 60% for room and pillar. Longwall produced 112.8 million short tons in the US in 2024, about 55% of underground coal (EIA). Alabama, Colorado and Illinois all report longwall output in the same table.

The trade-off is planned ground movement. Because the roof is meant to fall, the land above can sink, and subsidence can change how water moves through soil and streams. That makes surface ownership and water surveys part of the method decision, not an afterthought.

3. Open and longhole stoping: steep ore in strong rock

Crews drill long holes from one tunnel into the ore, blast it, and collect the broken rock from a lower level. The open stope stands by itself if the rock is strong, or it can be backfilled later. Backfill is usually a mix of cement with rock, sand or tailings (the ground waste from processing). Filling allows the neighboring stopes to be mined too, so less ore is left behind.

The method works at depth. LaRonde’s expansion reached 3,008 m in 2016, cited as the deepest longhole open stopes in the world at that date.

4. Cut and fill: selective mining of irregular ore

Miners take a horizontal or gently inclined slice of ore, fill the void with waste rock, sand or tailings, then stand on the fill to mine the next slice. Cut and fill is expensive but selective, with low ore loss and dilution. Dilution is waste rock that gets mixed into the ore and lowers its grade. Choose it when ore is steep or irregular, the host rock is poor, or the grade is high enough to pay for careful work.

5. Sublevel caving: Kiruna iron ore

Crews drive tunnels through the ore at regular levels, drill fans of long holes upward, and blast. The broken ore is pulled out while the waste rock above caves in behind it. LKAB says its underground mines use sub-level caving, and calls Kiirunavaara and Malmberget the world’s two largest underground iron ore mines. Kiruna’s main haulage level was 1,365 m below the ore outcrop as of 2020.

Caving breaks through to surface over time. A site with a town, road or rail line above the ore must plan for that long before production starts.

6. Block and panel caving: Oyu Tolgoi and Cadia

A block cave undercuts a very large mass of ore, and gravity does most of the breaking. Broken ore falls into funnels called drawbells and is hauled away from a production level below. It suits massive, steeply dipping, typically low-grade ore bodies that break up easily.

Rio Tinto says Oyu Tolgoi’s underground mine uses block caving, with about 200 km of tunnels, and that sustainable underground production started in March 2023. In Australia, Cadia, operated by Newmont, runs an automated production level in Panel Cave 1. Caves need heavy spending on tunnels before the first ore arrives, so they usually only suit very large deposits.

7. In situ leach: uranium without a shaft

In situ leach (ISL, also called in situ recovery or ISR) pumps a leaching solution through injection wells into the ore. Extraction wells then bring the metal-bearing liquid to a plant. There is no shaft, stope or rock hauling. According to the World Nuclear Association, suitable deposits are permeable sand or sandstone, confined above and below by impermeable layers and sitting below the water table.

ISL’s share of world uranium mining rose from 16% in 2000 to 56% in 2022, and it was 55% in 2024 (World Nuclear Association). Kazakhstan’s Karatau (Budenovskoye 2) produced 3,299 tonnes of uranium in 2024, and Inkai sites 1-3 produced 2,992 tonnes.

8. Jet boring and ground freezing: Canadian high-grade uranium

Some ore is too wet and weak for normal mining, yet rich enough to pay for special methods. Cameco mines Cigar Lake 480 m below surface with a jet-boring technique developed for that deposit. At McArthur River, brine circulates to keep the porous rock around the ore frozen, with mining between 530 and 640 m below surface. In 2024 the two sites produced 6,501 and 7,808 tonnes of uranium, the top two mines in the World Nuclear Association’s table.

How deep subsurface mines go

Depth affects rock stress, heat, hoisting time and cost. The figures below use different bases, as each source reports them, so treat the chart as a sense of scale rather than a strict ranking. Mponeng in South Africa reaches over 4 km in the Witwatersrand gold basin. Kidd in Ontario bottoms at about 3,014 m. Creighton’s No. 9 shaft, finished in 1969, is 7,138 feet deep, or about 2,176 m.

Chart

How deep six underground mines reach

Metres below surface; basis varies by mine

Depth alone does not set the method. Kiruna and Oyu Tolgoi cave large ore bodies, while Mponeng works a thin, tabular gold reef (a sheet-like layer of ore).

Surface or underground: what the data shows

Most mines start with the same question: is the ore close enough to the surface to reach by removing the rock above it? The EIA notes that underground mining is used when coal lies more than 200 feet below the surface, and that surface mining costs less. In 2024, surface mines supplied 305.8 of the 512 million short tons mined, about 60% (EIA).

Chart

US coal sales price by mine type

US$ per short ton, 2023 and 2024

Chart

World uranium output by mining method

Share of production, 2024

Underground coal sold for much more in 2024: US$85.88 per short ton against US$29.61 from surface mines. Do not read that gap as a pure method effect. Coal type also differs, because much surface output is lower-value Wyoming coal, and underground output includes higher-value eastern coal. The lesson still holds: an underground mine needs a richer or more valuable product to pay for itself.

A mine can also change method over its life. Creighton began as an open pit in 1901 and moved underground from 1906, and Oyu Tolgoi combines an open cut with its underground cave.

How to choose a subsurface mining method

The size, shape, orientation and type of the ore body set the short list, then rock strength narrows it. Work through these checks in order:

1. Shape and dip. Flat and layered points to room and pillar or longwall. Steep and narrow points to stoping or cut and fill. Very large points to caving.
2. Rock strength. Strong rock can stand open. Weak rock needs fill, or a method that lets it cave on purpose.
3. Grade and value. Low-grade ore needs high volume and low cost per tonne. High-grade ore can pay for selective work.
4. Permeability and water. Permeable ore between tight layers may suit leaching. Wet, weak rock may need freezing or grouting.
5. What sits above. Longwall and caving move the surface. Towns, rivers and heritage sites can rule them out, or require pillars that leave ore behind.
6. Ownership and permits. Confirm the license covers underground rights, the rules on ground movement, and groundwater approvals for any leaching.

The ore body type drives most of this, and our guide to types of ore deposits covers the common forms.

▶ What a block model is: the 3D grid under every modern mine plan. · 5:23

Subsurface mapping techniques that guide the method

None of those checks can be done without a picture of what lies underground. Each mapping technique measures one physical property, and no single one gives the whole answer. How deep a survey sees depends on the rock, the equipment and the survey layout, so ask a geophysicist to model the target before you buy a survey.

Some notes from the agencies behind these methods. Geoscience Australia describes magnetics as a non-invasive measure of perturbations caused by magnetic minerals in crustal rocks, and gravity as a way to map rock types and structure through density. The US EPA warns that a spread of small shallow masses can produce the same gravity effect as a large mass at depth, so gravity needs outside control on density. That also answers a common question about mapping how mass is distributed underground: gravity is the direct measure, but it is rarely unique without drill data.

The EPA lists mapping the bedrock surface, faults, sink holes and abandoned mines among seismic refraction goals. It also presents resistivity as a reconnaissance method whose anomalies are followed up with other geophysics or drill holes. For radar, penetration can reach about 15 m in dry sand or massive dry rock, but only a few centimeters in moist, clay-rich soil. That rules it out as a deep ore tool.

Kidd shows how these methods combine. An aerial geophysical survey in 1959 flagged the anomaly, and ground EM followed in 1963 before drilling confirmed the deposit.

▶ Remote sensing for mining, explained in plain words. · 2:01

How mapping improves confidence in mining targets

Confidence comes from independent measurements agreeing, then drilling testing that agreement. A sensible order for early-stage field decisions runs like this:

Infographic

From desk study to drill test in 5 steps

1. 1Desk studyGather old maps, logs and public survey data.
2. 2ScreenRank ground with satellite and regional airborne data.
3. 3SurveyMeasure the property that separates ore from host rock.
4. 4DrillTest the model and log the core.
5. 5UpdateRemodel, then continue or stop.

1. Desk study. Collect old maps, drill logs and government survey data for the license.
2. Screen from above. Use satellite imagery and regional airborne data to rank areas and spot access problems.
3. Survey the best ground. Pick the physical property that actually differs between the ore and its host rock.
4. Drill to test the model. Compare the main drilling types, then log core samples for grade, structure and rock strength.
5. Update and repeat. Remodel after each round. Stop spending on a target if results stop agreeing.

Newer tools, such as muon tomography, fit at the survey stage. On speed, gravity and magnetic crews can often mobilize quickly, but ask any provider about terrain corrections and data quality, because these decide whether a fast survey is usable.

Satellite screening sits at step two. Farmonaut’s satellite-based mineral detection reports turn a license boundary into ranked exploration targets, which sit ahead of any resource estimate and still need ground checks and drilling. For a scoped proposal, use the mining query form or contact the team.

▶ How to map a mining concession step by step in Farmonaut Mining Pro. · 6:03

Break-even grade for an underground mine

Break-even grade (also called cut-off grade in its simplest form) is the grade at which the metal in a tonne of ore just pays for mining and processing that tonne. An underground mine usually needs a higher figure than a pit, because each tonne costs more to reach. Use the calculator below with your own study costs. It covers operating costs only, so it leaves out the capital needed for shafts, ramps and plant.

Frequently asked questions

What is subsurface mining?

It is the extraction of ore from below ground, through shafts, ramps, tunnels or wells, without removing the rock above it. Surface mining removes that rock.

What are examples of subsurface mining?

Room and pillar, longwall, open stoping, cut and fill, sublevel caving, block caving and in situ leach. Real cases include Kiruna in Sweden, Oyu Tolgoi in Mongolia and Cigar Lake in Canada.

How can I use subsurface mining in a sentence?

“The ore lay too deep for a pit, so the company chose subsurface mining and built a ramp and a shaft.”

What minerals are mined underground?

Coal, potash, gold, copper, zinc, nickel, iron ore and uranium are all mined underground at the operations named in this guide. Salt, limestone and gypsum are too.

Is subsurface mining the same as underground mining?

Usually, yes. In situ leach is the main exception some people draw, because it recovers metal below ground through wells with no one working underground.

What is the difference between longwall and room and pillar mining?

Longwall removes a whole panel of a seam and lets the roof fall behind. Room and pillar leaves columns of ore in place to hold the roof, so less ore is recovered, but the surface above is less likely to sink.

What is the deepest mine in the world?

Mponeng in South Africa is the usual answer, at over 4 km below surface.

When does a project switch from open pit to underground?

When the cost of removing more waste rock per tonne of ore goes above the cost of mining that ore from below. A real decision also weighs the capital needed and the ground stability of the pit.

Which subsurface mapping technique should I start with?

Match it to the property that sets ore apart from its host rock: magnetics for magnetic minerals, gravity for dense bodies, EM or IP for conductive or sulfide-bearing rock. Start with existing government data, because it may already cover your area.

Can satellites map the subsurface?

Not directly. Optical satellites read the surface. They help by mapping alteration minerals and structure at surface that can point to what lies below. Drilling must confirm any target.

Does underground mining cause subsidence?

It can. Longwall and caving are designed to let the rock above collapse, and retreat mining removes pillars. Mines plan for this with surface monitoring and by leaving protective pillars where needed.