The complex dynamics of smoldering ground fires in wetlands

When we think of fire, we picture towering flames, roaring heat, and smoke-filled skies. But across the globe—and often beneath our very feet—another type of fire burns out of sight, quietly transforming ecosystems, endangering lives, and accelerating climate change. From the slow smolder of wetlands to the blazing heart of coal seams, underground fires represent one of the planet’s most persistent and overlooked hazards.

Smoldering wetlands

Wetlands are typically seen as natural firebreaks—lush, waterlogged, and resistant to flame. But this perception is increasingly outdated. Research shows that wetlands, particularly those rich in organic soils like peat and muck, are not immune to fire. In fact, under drought conditions, these soils become highly flammable and ignite into smoldering ground fires—a form of combustion that burns low and slow beneath the surface.

Smoldering is a flameless process where oxygen reacts directly with the surface of solid fuels. Unlike wildfires that pass quickly through a landscape, smoldering fires can persist for weeks, months, or even longer. The low temperatures (500–700°C) may seem mild compared to the 1800°C flames of traditional fires, but the prolonged heat exposure can cause just as much, if not more, ecological and structural damage.

In places like the southeastern United States—where wetland ecosystems are adjacent to fire-prone uplands—these ground fires are surprisingly common. Yet they remain difficult to detect and even harder to extinguish. Dried peat is hydrophobic, repelling water and rendering traditional firefighting methods nearly ineffective. Suppression efforts using chemicals or mechanical trenching come with high ecological costs and often limited success.

Human and environmental hazards

Smoldering fires don’t just pose ecological threats—they also have serious implications for public health and safety. They emit PM2.5, a class of fine particulate matter small enough to penetrate deep into the lungs, posing significant risks to cardiovascular and respiratory systems. Unlike surface wildfires that follow diurnal weather patterns, smoldering fires burn continuously, releasing smoke that lingers in low-lying areas, reducing visibility, and increasing the likelihood of nighttime vehicle accidents.

Perhaps more concerning is the role of ground fires in carbon emissions. Wetland soils store carbon accumulated over centuries. When these organic layers burn, they release vast quantities of CO₂—contributing disproportionately to greenhouse gas emissions. Though wetlands cover only 2–3% of Earth’s surface, they may store up to one-third of the planet’s terrestrial carbon. Ground fires in these regions can emit 75% more carbon per hectare than aboveground wildfires.

Please also read: Forest and Fire

Ecological consequences

The ecological impacts of smoldering fires are complex. Trees in wetlands often die when smoldering fires consume the organic soil around their roots. The phenomenon is especially severe under tree canopies, where drier soils and shrinking peat layers create air gaps that fuel combustion near root systems.

However, not all outcomes are negative. In cypress swamps, moderate-intensity fires may help maintain ecosystem balance by killing off competing species, allowing cypress trees to remain dominant. In other cases, smoldering fires can alter hydrology by lowering soil elevation and extending hydroperiods. This can transform forested wetlands into open marshes—habitats that support endangered species like the Florida panther and wood stork during dry seasons.

These microtopographic changes can also affect water storage across entire landscapes, reshaping how and where water is retained. While often seen as destructive, some of these changes could enhance the resilience of ecosystems facing increased climatic variability.

Earth’s longest-burning Infernos

Parallel to smoldering peat fires are coal seam fires—another class of underground infernos with even more dramatic consequences. Found in coal-rich regions from Pennsylvania to China, these fires can burn for decades or even millennia. The oldest known, located in Australia’s Burning Mountain, has been active for over 6,000 years.

Most coal seam fires are human-induced, often ignited during mining or waste disposal. Others, like Germany’s Brennender Berg, likely began through spontaneous combustion under specific pressure and oxygen conditions. These fires are notorious for the dangers they pose—collapsing roads, forming fiery sinkholes, and releasing toxic gases like carbon monoxide, mercury, and methane.

Coal seam fires also contribute significantly to global greenhouse gas emissions, although their exact impact is still under-researched. Estimates suggest they may consume between 20 million and 600 million metric tons of coal annually, a staggering loss of a non-renewable resource. Their detection often relies on satellite data, as the fires usually remain hidden until they breach the surface or trigger visible landscape changes.

Extinguishing these fires is notoriously difficult. Techniques like excavation, water injection, or clay encasement are expensive, disruptive, and often only temporarily effective. In some cases, such as Centralia, Pennsylvania, entire communities have been relocated at the cost of millions rather than continuing futile suppression efforts.

Toward a deeper understanding and smarter management

Despite the growing awareness, research on underground fires lags behind that of more visible surface fires. Much of the current knowledge comes from studies in northern peatlands or tropical regions. In contrast, wetland systems in the southeastern U.S. and similar biomes remain understudied—despite being highly vulnerable due to population growth, water demands, and climate pressures.

To manage and mitigate the risks, future efforts should focus on:

• Scaling up lab-based findings to landscape-level applications.
• Modeling long-duration fire behavior in heterogeneous environments.
• Evaluating how fire-altered topography affects vegetation and water systems.
• Understanding wildlife responses to changes in hydroperiod and habitat.
• Assessing the effectiveness and ecological impact of fire suppression techniques.

Crucially, a balanced perspective is needed. Not all underground fires are purely destructive. In some cases, they may serve ecological functions—such as regulating vegetation, supporting biodiversity, or maintaining critical wetland water levels. Recognizing when to fight and when to adapt will be key for land managers and conservationists alike.

Conclusion

From the smoldering depths of wetland peat to the ancient coal veins beneath mountains, underground fires remind us that not all environmental change happens in plain sight. These hidden blazes are agents of both devastation and transformation, challenging our traditional notions of fire, ecology, and climate.

As droughts intensify and human pressures mount, these fires will likely become more frequent, more damaging, and more consequential. To face this future, we must invest in research, embrace complexity, and approach fire not merely as a threat to extinguish—but as a force to understand, respect, and, where possible, coexist with.