How do underground fires keep burning after a wildfire is out?

Numbers match the citations in the article. A working link isn't proof that a page supports a claim; check the quoted passage and date.

1. 1

   Smouldering (Wikipedia)

   WikipediaPublished Oct 7, 2026Checked Oct 11, 2026

   “Smouldering (British English) or smoldering (American English; see spelling differences) is the slow, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel. Many solid materials can sustain a smouldering reaction, including coal, cellulose, wood, cotton, tobacco, cannabis, peat, plant litter, humus, synthetic foams, charring polymers including polyurethane foam and some types of dust. Common examples of smouldering phenomena are the initiation of residential fires on upholstered furniture by weak heat sources (e.g., a cigarette, a short-circuited wire), and the persistent combustion of biomass behind the flaming front of wildfires.”

2. 2

   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.”

3. 3

   Characterising the holdover phase of lightning-ignited wildfires in Catalonia

   Agricultural and Forest Meteorology. (Pineda et al.)Published Sep 1, 2022Checked Oct 11, 2026

   “The bulk of lightning-fire ignitions (80%) occurred between June and September during the warm season. Conifer forests concentrate almost half of the lightning-ignited wildfires. Then, we spatially interpolated air temperature and relative humidity data from automatic weather stations to calculate a weather index describing the evolution of fuel moisture content at the specific wildfire location, from the time of the lightning-caused ignition to the time of the fire detection. Results showed that fuel moisture content drives lightning-ignited wildfires since most ignitions around midday turn into flaming combustion almost immediately, when fuel moisture content reaches the minimum of the day. The holdover duration increased in late afternoon lightning-ignited fires, which remain smouldering overnight and evolve to flaming combustion in the next solar cycle. We found that latent fires above 24 h were rare (15%), and only 3% of the fires had a holdover period above three days. Only 1 in ∼840 cloud-to-ground flashes started a wildland fire. Our outcomes provide valuable insight to improve the modeling and management of natural wildfires in the Mediterranean areas.”

4. 4

   Peat fires contribute disproportionately to Siberian fire carbon emissions.

   Science advances (Khairoun et al.)Published Mar 18, 2026Checked Oct 11, 2026

   “Arctic and boreal fires are critical threats to terrestrial carbon reservoirs, particularly peat fires that trigger long-term irrecoverable carbon losses and permafrost thaw. However, the occurrence of peat fires and their associated carbon emissions remain highly uncertain. 30-meter satellite-derived maps of burned area and peatland coverage reveal that Siberian fires burned over 107 million hectares during the 2001 to 2023 period, with peat fires accounting for up to one-third of this area. These peat fires emitted 1.24 ± 0.06 petagram of carbon, largely exceeding conventional datasets' estimates. We found that anomalous dry and warm climatic conditions represent the primary driver of extreme peat fire seasons and that overwintering of 2020's late-season peat fires substantially contributed to extensive fires of 2021. Peat fires, especially those in Arctic regions, exhibit a pronounced sensitivity to extreme weather, posing a critical threat to the stability of permafrost peatlands and their large carbon stocks.”

5. 5

   Impact of Fire and Heat Stress on Soil Microorganisms: A Review of Community Changes, Molecular Responses and Plant-Beneficial Roles.

   Environmental microbiology reports (Orozco-Mosqueda et al.)Published Apr 1, 2026Checked Oct 11, 2026

   “This review examines the effects of fire on soil-beneficial microbial communities, with particular attention to how surface and underground fires drive shifts in microbial diversity and functional roles within the agroecosystems. These changes impact key processes such as nutrient cycling, soil physicochemical structure and organic matter decomposition, ultimately affecting crop production. Bacterial groups such as Firmicutes and Actinobacteria often increase in abundance following fire events, while others lacking survival strategies tend to decline. Resilient fungal groups, including Ascomycota (such as Aspergillus, Penicillium and Trichoderma), frequently play pivotal roles during the recovery process. Fire can also enhance microbial metabolic activity, particularly in pathways involved in organic matter degradation, leading to short-term increases in nutrient availability that support plant regrowth. Finally, the review discusses the molecular responses of microbes to fire and outlines perspectives for better understanding this type of stress and how it affects the beneficial soil microbiota in agricultural edaphic systems.”