A lightning bolt that hit U Minh Ha forest on 21 September set off a peat fire that smouldered underground for 19 days, researchers reported Friday. Surface flames were put out within hours, but heat sensors later traced a burning front 410 metres (1,345 feet) through the peat, reaching depths of 1.8 metres (5.9 feet).
The finding, published by the fictional Mekong Fire Ecology Lab in the Journal of Delta Carbon, challenges the assumption that heavy rain and extinguished flames mean a peat fire is over. U Minh Ha, in Ca Mau province, is a wetland forest in Vietnam’s far south, where layers of partly decayed plants hold carbon accumulated over centuries.
The fire beneath the rain
Researchers began checking the site after a thermal sensor registered heat under ground that appeared cool from above. By then, 74 millimetres (2.9 inches) of rain had fallen in four days and the visible fire had been out for more than a week. The team found a network of hot pockets beneath a 36-hectare (89-acre) patch of forest.
Karen Ulrich, a fire ecologist who led the field team, said the strike had ignited dry peat below a thin layer of damp soil. “We had a wet surface, no smoke column and no flame. The ground was still consuming itself underneath us,” she said. The team estimated the affected peat released about 18,000 tonnes of carbon dioxide.
We had a wet surface, no smoke column and no flame. The ground was still consuming itself underneath us.
Karen Ulrich, fire ecologist
The researchers say that is not evidence that every lightning strike can start a long-running fire. But peat in three of 29 test plots was dry enough to burn below the surface, despite vegetation and soil that looked wet. The team says routine patrols, which focus on smoke and visible flame, could miss fires until they have spread well beyond the original strike.
A warning for wetland managers
Dominic E. Harrison, a climate modeller at the Southern Wetlands Field Institute, said the episode exposes a blind spot in regional fire forecasts. Models generally treat rain as a strong brake on peat fires, he said, but do not account for heat travelling through deeper, drier layers. “The rain can hide the evidence before it has stopped the process,” he said.
The distinction matters because peat fires can release carbon stored in the ground over long periods, while damaging plant roots and making the soil more prone to drying. The team’s estimate covers only the measured site and is not a forecast for the whole forest. Still, Harrison said, the strike showed how a small ignition can produce emissions that surface observations fail to register.
Henrieta Lubina, a wetland scientist who was not part of the field team, said officials should add subsurface temperature checks after lightning storms in peat areas. That would require more sensors and field staff, and the researchers have not yet tested a monitoring system across a full dry season. The lab plans to return in November to compare the burned patch with nearby plots.
For now, the team is urging local forest managers to treat a quiet, rain-soaked site as unconfirmed rather than safe after a peat fire. The study’s larger warning is less about one extraordinary bolt than about what it revealed: in a forest that looks extinguished, the most consequential part of a fire may be the part nobody can see.
