An underground fire is a combustion event that occurs entirely beneath the soil surface, unlike a traditional wildfire. These subterranean blazes, often called ground fires, are defined by the fuel they consume and the limited oxygen environment in which they burn. Unlike rapidly spreading surface fires fueled by grasses and brush, ground fires consume denser, buried organic material. This allows them to persist out of sight for extended periods, sometimes for decades or centuries.
Fuel Sources and Ignition
The primary fuels sustaining an underground fire are dense, carbon-rich materials found in the subsurface. The most common of these are peat, which is partially decayed vegetative matter, and deep, extensive coal seams. Other organic material, such as the extensive root systems of trees and dried layers of humus and duff, can also serve as fuel sources, especially in forest or wildland environments.
These fires can be ignited by a variety of thermal sources, often starting above ground before moving into the subsurface. A common cause is a surface wildfire that burns long enough to heat and ignite underlying peat or coal outcrops. Ignition can also occur naturally through lightning strikes or via spontaneous combustion. Spontaneous combustion occurs when coal or finely divided organic matter slowly oxidizes, causing the material’s temperature to rise until it reaches its ignition point without an external spark. Human activity, such as abandoned mine workings, mining accidents, or burning trash in landfills built over coal deposits, is another frequent trigger.
Unique Burning Characteristics
The behavior of an underground fire is fundamentally different from a flaming surface fire because it exists in a low-oxygen environment. This limited air supply forces the combustion to take the form of smoldering, a flameless, slow burn that produces smoke and heat rather than visible flame. Smoldering allows the fire to endure for long periods, as the fuel is consumed at an extremely slow rate.
The fire’s propagation speed is remarkably slow, sometimes moving as little as one millimeter per minute in dense peat. Despite this slow pace, the temperatures achieved deep underground are intensely high and sustained, often exceeding 1,000 degrees Fahrenheit. The surrounding earth acts as an insulator, trapping the heat and preventing the fire from being extinguished by rainfall or seasonal changes. This thermal insulation and slow, steady consumption of buried fuel mean that some coal seam fires have been burning continuously for thousands of years.
Environmental and Safety Impacts
Long-term subterranean combustion poses serious hazards to the environment and human safety. As the buried fuel is consumed, the material supporting the ground disappears, leading to significant land instability. This results in ground subsidence, surface cracking, and the formation of dangerous sinkholes that can collapse roads, infrastructure, and buildings without warning.
The smoldering process releases a continuous plume of toxic gases through fissures and vents in the earth. These emissions include significant quantities of carbon monoxide, sulfur oxides (SOx), nitrogen oxides (NOx), and methane. Additionally, the burning releases heavy metals like mercury and other particulates that contaminate the air, contributing to regional air pollution and health problems. The intense heat also sterilizes the soil above the fire, resulting in long-term ecological damage and preventing vegetation from taking root.
Detection and Suppression Challenges
Detecting an underground fire is challenging because it is largely invisible; the only surface indicators are subtle heat signatures, wisps of smoke, or the chemical composition of venting gases. Specialized tools are required to accurately locate the extent of the burn zone. Detection methods rely on thermal imaging, gas sensors that monitor for elevated levels of combustion byproducts like carbon monoxide, and boreholes to measure subsurface temperatures and gas concentrations.
Extinguishing these fires is notoriously difficult and costly, as the fire must be starved of fuel, oxygen, or heat. For shallow fires, the most reliable method is complete excavation, where the burning material is dug out, quenched, and then buried. For deeper fires, suppression techniques focus on sealing the fire from oxygen, such as injecting inert gases like nitrogen or carbon dioxide into boreholes. Extensive flooding with water is often temporary, as residual heat can cause the fire to reignite. Another technique involves creating isolation trenches or cutoff walls to physically separate the fire from its unburned fuel source.