What Is an Underground Fire and How Do They Start?

An underground fire is a combustion event that burns beneath the earth’s surface, typically in coal seams or layers of peat, consuming fuel slowly and without visible flames for months, years, or even centuries. These fires smolder at relatively low temperatures compared to open flames, fed by oxygen that seeps through cracks, mine shafts, and porous soil. They start through a mix of natural processes and human activity, from the spontaneous heating of coal exposed to air to careless waste burning near exposed seams. What makes them remarkable is their persistence: once established, an underground fire is extraordinarily difficult to stop.

How Underground Fires Burn

Underground fires are fundamentally different from the blazes you see on the surface. Rather than producing open flames, they sustain themselves through smoldering, a slow, flameless form of combustion that relies on direct chemical reactions between oxygen and solid fuel. Peat and coal are both porous enough to allow air to reach the burning front, and both contain enough carbon to keep the reaction going indefinitely as long as oxygen keeps arriving. Research on peat smoldering has found that propagation can continue at remarkably low oxygen levels, potentially below two percent, provided the airflow rate is sufficient. When oxygen supply drops too low, smoldering still persists at a minimum temperature of roughly 300 °C but stops spreading outward.¹

This is why underground fires are so stubborn. A surface wildfire burns hot and fast, consuming available fuel and moving on. A smoldering underground fire barely needs anything. It creeps through fuel beds at speeds measured in centimeters per day, throwing off heat that dries out adjacent material and primes it to burn next. The temperatures involved are modest by combustion standards but devastating underground, where the heat has nowhere to dissipate quickly and instead bakes the surrounding geology.

Natural Ignition and Spontaneous Combustion

One of the most counterintuitive ways underground fires start is through no external spark at all. Coal exposed to air undergoes a slow oxidation process that generates heat. If that heat cannot escape, perhaps because the coal is buried under rubble or piled in a thick seam with poor ventilation, temperatures climb. Laboratory studies tracking this process in coal samples have mapped out distinct stages: a latent period where heat builds invisibly, a slow heating phase where water in the coal evaporates, an accelerated phase once the moisture barrier is gone, and finally outright combustion. In tested coal samples, this progression from ambient temperature to ignition took roughly 35 to 42 days.²

Moisture plays a crucial gatekeeper role. During the slow heating phase, much of the energy generated by oxidation is absorbed by water evaporating from the coal. This buys time. But once the coal dries out, temperatures climb much faster, and the process enters a runaway phase that is hard to reverse. The same principle applies in peat fires: laboratory experiments show that when peat moisture drops below about 50 percent on a dry-mass basis, smoldering fire consumes nearly everything in its path, turning plant roots to ash regardless of their thickness. Above that moisture threshold, more roots survive with varying degrees of charring.³

Lightning strikes, surface wildfires burning downward into organic soils, and even geothermal heat in volcanically active areas can provide the initial spark in natural settings. But spontaneous heating of coal is a documented and well-studied trigger that requires no external ignition source at all.

Human Causes

Most underground fires in coal regions trace back to some form of human activity. Mining opens up coal seams to air, creating the ventilation pathways that oxidation needs to build toward ignition. Abandoned mines are especially dangerous because they leave behind exposed coal faces, fractured rock that channels airflow, and nobody watching. Underground mine fires and burning waste piles ignited by human error or negligence are responsible for a wide range of consequences, from atmospheric pollution and acid rain to land subsidence, habitat destruction, and respiratory disease in nearby communities.⁴

The triggers are often mundane. Trash burning near exposed coal outcrops, welding or cutting work in active mines, discarded cigarettes, and improperly managed waste dumps have all been documented starting points. In developing countries where small-scale coal extraction happens with minimal safety infrastructure, fires sometimes start from cooking fires built directly on coal-bearing ground. The common thread is that coal, once exposed and given air, needs only a modest amount of heat to begin the slow climb toward self-sustaining combustion.

The Self-Reinforcing Feedback Loop

What turns a localized hot spot into a fire that burns for decades is a feedback mechanism between heat, fracturing, and oxygen supply. As an underground fire heats the surrounding rock, thermal stress causes that rock to crack. Gravity pulls weakened material downward, creating new fractures and voids. These fractures act as chimneys, drawing fresh air from the surface down toward the burning front. Numerical simulations of this process show that oxygen in the active high-temperature zone drops to around 4.4 percent as it is consumed, while fracture channels feeding that zone maintain oxygen levels between 15 and 18 percent, continuously resupplying the fire.⁵

This is the cycle that makes underground fires so persistent: burning creates cracks, cracks deliver oxygen, oxygen sustains burning, and burning creates more cracks. Each round of this loop extends the fire’s reach and strengthens its air supply. Trying to smother the fire by sealing surface vents often fails because the fire simply fractures new pathways to the surface. It is a self-reinforcing system that, once mature, resists most conventional firefighting approaches.

What Happens Above Ground

The surface effects of an underground fire can take years to become visible, which is part of what makes them so dangerous. The most dramatic consequence is subsidence: as the fire consumes coal or peat below, the ground loses structural support. The results include sinkholes, trenches, broad depressions, and in severe cases, large-scale collapses where whole sections of terrain drop.⁶ Roads crack and buckle, foundations shift, and utility lines snap. In areas where people live, this creates genuine hazards: a sinkhole opening beneath a street or backyard can expose superheated ground and toxic gases with little warning.

Toxic emissions are the other major surface concern. Uncontrolled coal fires release carbon dioxide, methane, carbon monoxide, mercury, and a range of other harmful substances into the atmosphere.⁷ These pollutants vent through cracks and fissures, often concentrated in plumes that can make the immediate area hazardous to breathe. In coal-mining towns, the slow accumulation of carbon monoxide in basements and the appearance of warm, sulfurous steam rising from the ground have been early warning signs that a fire is approaching.

The Scale of Greenhouse Gas Emissions

Underground coal fires collectively represent a significant and often overlooked source of greenhouse gas emissions. A study estimating emissions from smoldering coal fires in China put the annual output at roughly 6.5 to 42 million tons of greenhouse gases, depending on smoldering conditions. The research found something particularly worth noting about the composition of those emissions: as smoldering temperatures rose, the share of the greenhouse effect attributable to carbon dioxide actually decreased, falling from about 34 percent to around 24 percent. Meanwhile, the contribution from precursor gases like methane and carbon monoxide grew from about 66 to 76 percent of the total warming impact.⁸

This matters because methane is a far more potent greenhouse gas than carbon dioxide over shorter time horizons. Smoldering fires, by their nature, produce incomplete combustion, which favors methane and carbon monoxide over the carbon dioxide that complete burning would generate. The same study estimated that smoldering produces between 18 and 41 percent of the greenhouse emissions that fully burning the same amount of coal would generate. That sounds like a partial discount, but the fires are uncontrolled, unmeasured, and ongoing, so even partial emissions at that scale add up year after year.

Centralia, Pennsylvania

No discussion of underground fires is complete without Centralia, the small Pennsylvania town that became a cautionary tale. In 1962, residents set fire to a landfill near the edge of town. The burning refuse ignited the Buck Mountain coal bed beneath, and because the fire started in the hinge zone of a geological fold separating two synclines, it had room to spread in four directions simultaneously.⁹ Multiple attempts to contain the fire failed. Trenches were dug to cut it off; the fire burned past them. Boreholes were drilled to flood the fire zone with water and slurry; the fire moved deeper. By the 1980s, sinkholes were opening in yards, carbon monoxide was seeping into homes, and steam laced with toxic gases was venting through cracks in roads.

The federal government eventually relocated most of Centralia’s residents and condemned the town. As of the early 2000s, the fire was still burning out of control, having destroyed the physical structure of an entire community and displaced its population.¹⁰ Centralia is often described as a worst-case scenario, but fires of comparable scale burn in coal regions across the globe. Centralia is simply the one that happened in a visible American town and became impossible to ignore.

The Cost of Fighting Them

Underground fires are expensive precisely because they resist cheap solutions. In the United States, the combined cost of coal-fire remediation projects completed, budgeted, or projected by the federal Office of Surface Mining Reclamation and Enforcement exceeds one billion dollars, with about 90 percent of that concentrated in Pennsylvania and West Virginia. Across the country, 15 states have cumulative coal-fire project costs above one million dollars, and the pattern is clear: underground fires cost far more to address than surface ones.¹¹

One approach that has shown promise involves injecting cellular grout, a foam-containing mixture of cement, fly ash, and special additives, directly into the fire zone. The grout is designed to be highly flowable so it can penetrate cracks and voids underground, and heat-resistant enough to survive the fire environment. The strategy attacks all three elements of the fire: it physically covers the fuel to prevent further oxidation, it seals cracks and vents to cut off oxygen, and its mass absorbs heat. Surface application after strip mining can also prevent new fires from starting by shielding exposed coal from air.¹² Even with such techniques, though, many established fires are simply too large and too deep to extinguish economically. The practical response in many cases has been to monitor, try to contain the fire’s spread, and wait.

Overwintering Peat Fires

A category of underground fire that has drawn increasing attention from climate scientists is the “overwintering” or “zombie” fire. These fires burn in peat soils through the boreal winter, smoldering under layers of snow and frozen ground, and then re-emerge the following spring to ignite new surface fires before lightning season even begins. In eastern Siberia’s Yakutia region, overwintering fires accounted for about three percent of total burned area between 2012 and 2020. In 2020, a particularly severe year, they were responsible for roughly 7.5 percent of the annual burned area, about seven times the average of the preceding years.¹³

What makes these fires especially alarming is their resistance to winter conditions. Research simulating the effects of snowfall on shallow peat fires found that even heavy natural snowfalls, at intensities you would consider a serious winter storm, cannot suppress an established smoldering peat fire. The snow melts on contact with the heated ground, and the fire continues below the surface undisturbed.¹⁴ In a warming climate, where permafrost is thawing and exposing previously frozen peat to drying, the conditions for overwintering fires are expanding. Peatlands in the Arctic and boreal zones store enormous amounts of carbon, and fires in those soils release it directly into the atmosphere, creating a feedback loop between warming and further emissions.

What Underground Fires Do to Soil Ecosystems

Beyond the visible damage of sinkholes and smoky vents, underground fires reshape the living systems in the soil itself. Fire, whether it burns above or below the surface, significantly influences soil microbial communities by altering their composition, functional diversity, and the ecological services they provide to plants.¹⁵ Underground fires are especially destructive to soil biology because the heat persists for so long. A surface wildfire passes through in minutes, killing microbes in the top few centimeters but leaving deeper communities intact. A smoldering underground fire can bake the same patch of soil for weeks or months, sterilizing it to depths that surface fires never reach.

The practical consequence is that land recovering from an underground fire often faces a much longer road back to ecological health. Nutrient cycling slows when the microbial communities that drive it are destroyed. Plant roots cannot establish in sterilized or structurally collapsed soil. In peat systems, the organic soil itself is consumed as fuel, so there may be nothing left to recover into. Where coal fires have burned for decades, the surface terrain can be permanently altered: fused and baked rock, collapsed mine voids, and chemically contaminated ground that resists revegetation without extensive remediation.

Why They Are So Hard to Detect Early

One of the reasons underground fires grow so large before anyone intervenes is that they are genuinely difficult to find in their early stages. A fire burning 10 or 20 meters below the surface produces no visible smoke, no flames, and only subtle thermal signatures on the ground above. By the time residents notice warm patches, dying vegetation, or sulfurous smells, the fire may have been burning for years and spread across a wide area. Satellite-based thermal infrared imaging has become the primary tool for detecting these fires at scale, allowing researchers to identify surface temperature anomalies that hint at burning below. But the resolution and timing of satellite passes limit how early detection can happen, and underground fires in remote areas, particularly the boreal peatlands of Siberia and northern Canada, can burn unmonitored for entire seasons before anyone notices.

Ground-based detection relies on drilling boreholes and measuring subsurface temperatures directly, or on detecting the signature gases that coal produces at different stages of heating. The laboratory research on coal spontaneous combustion has identified specific gases that appear at predictable temperatures: ethylene appears around 85 °C, ethane around 115 to 130 °C, and acetylene around 180 to 195 °C.² Monitoring for these gases in mine ventilation systems or at surface vents can give early warning that coal is heating, potentially catching the process before it progresses to actual combustion. The challenge is that this works only where someone is actively monitoring, which excludes the vast majority of abandoned mines and natural coal outcrops worldwide.

Underground Fires Beyond Coal Country

While coal fires dominate the headlines, underground fires are not limited to mining regions. Peat fires are a growing concern in tropical countries like Indonesia, where drained peatlands burn during dry seasons and produce massive plumes of haze that affect air quality across Southeast Asia. These fires are driven by land-use change: peat swamps are drained for agriculture, the surface dries out, and fires set to clear land burn downward into the peat. Once in the peat, the fire can smolder for months, surviving rain events that extinguish the surface flames above.

Landfill fires represent another category of underground combustion. Buried organic waste decomposes and generates heat, and in poorly managed landfills, this can lead to subsurface fires that are strikingly similar to coal fires in their persistence and difficulty of suppression. The same principles apply: smoldering combustion in porous material, sustained by oxygen seeping through waste layers, producing toxic emissions and resisting water-based suppression. Municipal landfill fires have forced evacuations and created public health emergencies in countries around the world, though they receive far less scientific attention than coal or peat fires.

Even underground root systems can serve as fuel conduits. Research on peat fires has shown that smoldering underground fire can travel along root channels and re-emerge at the surface, initiating new flaming fires in vegetation above.³ This means a fire that appears to have been contained on the surface may still be alive underground, following root networks to pop up tens of meters away. Firefighters battling wildland fires in peat-rich areas have long known this behavior, and it is one of the reasons peat fires are considered among the most labor-intensive to fully extinguish.