How do underground peat fires keep burning after wildfires are out?

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1. 1

   Deep peat fire persistently smouldering for weeks: a laboratory demonstration

   International Journal of Wildland Fire (Qin et al.)Published Dec 7, 2022Checked Oct 11, 2026

   “Peatlands are becoming more vulnerable to smouldering fires, driven by climate change and human activities. This work explores the persistent burning, propagation, and emission of the deep peat fire. Laboratory experiments are conducted with a 1-m deep peat column, and smouldering fires are initiated at different depths. Key results We found localised burning and multi-directional smouldering fire spread in deep peat layers. The smouldering temperature first decreases with depths up to −40 cm (from around 550 to 350°C) and then remains at about 300°C in the deeper layers. High moisture content can slow down in-depth fire propagation and reduce the burning duration. Peat fire can burn in deep layers for weeks, and its combustion is incomplete with small mass loss, because of a limited oxygen supply and low smouldering temperature. Measuring the carbon monoxide concentration near the surface can detect underground fire and monitor its intensity. Implications This work helps reveal the underlying mechanism of the in-depth smouldering wildfires in peatland and supports future larger-scale peat fire experiments in the field.”

2. 2

   Persistent Underground Smouldering Fire in Deep Peat Layer

   Imprensa da Universidade de Coimbra eBooks (Qin et al.)Published Jan 1, 2022Checked Oct 11, 2026

   “Peatlands are important terrestrial carbon pools. Due to climate change and human activities, peatlands are more prone to large-scale fires than ever before, especially deep underground fires. However, most current smouldering researches focus on small-scale smouldering behaviour in relatively shallow layers, which poses a research gap. This work explores in-depth (up to 60 cm) smouldering behaviour, such as persistence, propagation and emission. Experimental results demonstrate that underground smouldering fires can sustain in deep soil layers for more than a week without any additional oxygen supply. Because of the competition of oxygen supply and heat losses, a critical depth of 55 cm for smouldering propagation was obtained, below which smouldering cannot self-sustained propagate. This work will help better connect lab-scale experiments with real underground smouldering peat fires and understand smouldering dynamics in deep soil layers.”

3. 3

   Smouldering wildfires in peatlands, forests and the arctic: Challenges and perspectives.

   Current opinion in environmental science & health (Rein & Huang)Published Dec 1, 2021Checked Oct 11, 2026

   “Wildfires can be divided into two types, flaming or smouldering, depending on the dominant combustion processes. Both types are present in most wildfires, and despite being fundamentally different in chemical and physical terms, one transitions to the other. Traditionally, science has focused on flames, while smouldering is often misinterpreted. But smouldering wildfires are emerging as a global concern because they cause extensive air pollution, emit very large amounts of carbon, are difficult to detect and suppress, and could accelerate climate change. Central to the topic are smouldering peat fires that lead to the largest fires on Earth. Smouldering also dominates the residual burning after flames have died out and firebrand ignition. Finally, smouldering is an important part of Arctic wildfires, which are increasing in frequency. Here, we present a scientific overview of smouldering wildfires, the associated environmental and health issues, including climate change, and the challenges in prevention and mitigation.”

4. 4

   Resurfacing of underground peat fire: smouldering transition to flaming wildfire on litter surface

   International Journal of Wildland Fire (Zhang et al.)Published Feb 4, 2024Checked Oct 11, 2026

   “Smouldering wildfires in peatlands are one of the largest and longest-lasting fire phenomena on Earth, but it is unclear whether such underground peat fires can resurface to the ground and ignite a flame on the litter layer. This work conducted a laboratory experiment by putting a 5-cm thick litter layer (banyan tree leaves with a density of 27–53 kg/m3) onto a 10-cm thick peat sample (moisture content of 10–100%). Key results and conclusions Tests confirmed that a smouldering peat fire, ignited at the bottom, can propagate upwards and resurface to ignite a flaming wildfire on the surface litter layer. The propensity of litter to be flaming ignited decreased with increasing peat moisture content and litter layer density. We found the threshold of such surface flaming as a function of temperature and temperature increase rate at the interface between peat and litter. Finally, large field experiments successfully reproduced and validated the laboratory observations. Implications This work reveals an important wildfire ignition phenomenon that has received little attention but may cause new spot fires, accelerate fire progression and exacerbate its hazards.”

5. 5

   Review of combustion issues in underground fires

   Research paper (Saulov et al.)Published Jan 1, 2013Checked Oct 11, 2026

   “Due to specific characteristics of smouldering, such as low temperatures and small propagation velocities, underground fires are difficult to detect, especially in their initial stages. For the same reasons, estimation of the extent of the detected fire is problematic. Underground fires are also extremely difficult to extinct. Such fires can last for a very long period of time, posing a serious safety threat and having substantial adverse environmental and economic consequences. In this paper, we review current understanding of several major combustion issues in underground fires, which originated from the specifics of smouldering processes, including fire detection and measurements as well as some approaches to control and extinct the fire. We also review environmental and socioeconomic impact of underground fires. Limited number of publications on the issues associated with smouldering, in general, and underground fires, in particular, is available. As a result, our understanding of the phenomena involved is incomplete. Further research in the area is required.”

6. 6

   Upward-and-downward spread of smoldering peat fire

   Proceedings of the Combustion Institute (Huang & Rein)Published Jun 21, 2018Checked Oct 11, 2026

   “Once ignited, the 1st-stage upward fire spread is initiated towards the free surface (opposed smoldering) with a peak temperature of 300 °C, leaving behind a char structure that does not collapse. Then, a 2nd-stage downward spread (forward smoldering) is activated with a peak temperature of 600 °C and regression of free surface. The upward spread is faster than the downward spread. The rates of both upward and downward spread decrease as the peat density or depth is increased. These experimental observations are successfully captured by a 1D computational model of heat and mass transfer with 5-step kinetics. Modelling results further suggest that (1) the oxygen diffusion controls the entire upward-to-downward spread of peat fire, (2) the oxidation of peat sustains the 1st-stage upward spread, and (3) the oxidation of char sustains the 2nd-stage downward spread. This is the first study investigating the upward spread of peat fire, which helps understand the persistence of peat fire and guide the fire prevention and suppression strategies.”

7. 7

   How drones are hunting fires hidden beneath the Cairngorms

   BBC NewsPublished Oct 10, 2026Checked Oct 11, 2026

   “Dramatic flames from the huge Cairngorms wildfire in July have disappeared - but hot spots can continue to burn beneath the earth.”