Underground coal fires are a persistent geological phenomenon, representing the slow, smoldering combustion of coal seams deep beneath the Earth’s surface. Unlike typical surface blazes, these fires can burn for decades, centuries, or even millennia, insulated from weather and surface intervention. They are zones of low-temperature oxidation that sustain themselves underground. Determining the precise global number of these fires is nearly impossible, underscoring the complexity of the issue.
Mechanism of Underground Coal Fires
An underground coal fire requires three components to ignite and sustain itself: fuel, an oxidizer, and an ignition source, often called the “fire triangle”. The fuel is the coal seam itself, and the oxidizer is oxygen, which seeps into the seam through fissures, boreholes, or exposed outcrops. The limited oxygen supply underground prevents rapid, flaming combustion, resulting in a slow-burning smoldering process.
These fires can be initiated by natural events, such as lightning strikes or wildfires that spread to an exposed seam. A major cause is spontaneous combustion, where the oxidation of coal, especially lower-grade coal or coal containing reactive minerals like pyrite, generates heat. If this heat cannot dissipate quickly enough, the coal’s temperature rises until it reaches the thermal runaway threshold, causing it to ignite. Human activity, particularly mining, often facilitates ignition by exposing coal to air, creating pathways for oxygen, or through accidents.
Once started, the fire is difficult to stop because the surrounding earth acts as an insulator, trapping the heat and allowing the fire to continuously feed on the coal. The combustion process creates a natural convection current, drawing fresh air into the fire zone through cracks and fissures and venting hot gases through the surface. This cycle allows the fire to persist until the fuel is exhausted or the flow of oxygen is completely cut off.
The Global Count and Geographic Distribution
Determining the exact number of underground coal fires worldwide is a challenge, but thousands are burning across at least 22 countries at any given time. Since they burn out of sight, scientists rely on specialized techniques, like satellite-based thermal imaging and gas analysis, to detect heat anomalies and chemical signatures on the surface. These remote sensing methods provide an estimate rather than a definitive count.
The majority of these fires are concentrated in a few major coal-producing regions globally. China has the most extensive problem, with historical estimates suggesting that between 10 and 200 million tons of coal are consumed by fires or made inaccessible for mining annually. The most severely affected provinces include Xinjiang, Inner Mongolia, and Ningxia, where over a hundred major fire areas have been identified.
India’s Jharia coalfield in the state of Jharkhand represents another major hotspot, where mine fires have been burning since 1916. Reports indicate that 68 to over 160 individual fires are active beneath this region, consuming the country’s limited reserves of prime coking coal. In the United States, the federal Office of Surface Mining lists over 100 fires burning beneath nine states. The Centralia, Pennsylvania, mine fire is the most famous example, having burned since 1962.
Environmental and Geological Consequences
The environmental impact of underground coal fires is substantial, releasing a broad range of atmospheric pollutants. The incomplete combustion occurring deep underground generates greenhouse gases, including carbon dioxide, methane, and carbon monoxide. These fires are also a major source of toxic emissions, such as sulfur dioxide, which contributes to acid rain, and trace elements like mercury, which contaminate local ecosystems.
Geologically, the consumption of the coal seam creates voids beneath the surface, leading to land subsidence and the formation of sinkholes. This ground instability poses a threat to infrastructure, including roads, pipelines, and buildings, and can necessitate the evacuation of entire communities. Furthermore, these fires destroy a valuable non-renewable resource, making millions of tons of coal inaccessible for future energy use.
Monitoring and Mitigation Challenges
Monitoring fires that can be hundreds of feet below the surface requires sophisticated and costly technology. Remote sensing is a primary tool, utilizing satellite and airborne thermal infrared sensors to detect the heat radiating from the underground fire through the soil. Scientists also use Differential Synthetic Aperture Radar Interferometry (DInSAR) to measure subtle changes in the ground surface, detecting the subsidence caused by the burning coal.
Extinguishing these fires is a complex, time-consuming, and expensive endeavor, with no single method guaranteeing success. One technique involves excavation, where the burning coal is physically dug out and quenched, but this is only feasible for shallower fires. For deeper seams, engineers may attempt to smother the fire by injecting inert gases like nitrogen or carbon dioxide into boreholes to starve the combustion of oxygen. Sealing the surface with clay, grout, or hydraulic barriers is also used to cut off the air supply and cool the seam, but the fire’s ability to find new oxygen pathways often causes these efforts to fail.