Peat fires can burn up to 15 feet deep and are extremely difficult to extinguish. Russia, which has been plagued by peat fires this summer, constructed a 30-mile-long water pipeline from the Oka River to an area with peat fires east of Moscow.
The excerpt below is from a New York times article and is a quote from Guillermo Rein, an expert on smoldering subterranean fires and an assistant professor at the University of Edinburgh in Scotland.
Smoldering fires, the slow, low-temperature, flameless form of combustion, are an important phenomena in the Earth system, and the most persistent type of combustion. The most important fuels involved in smoldering fires are coal and peat. Once ignited, these fires are particularly difficult to extinguish despite extensive rains, weather changes or firefighting attempts, and can persist for long periods of time (months, years), spreading deep (5 meters) and over extensive areas of forest subsurface. Indeed, smoldering fires are the longest continuously burning fires on Earth. The Burning Mountain, a coal deposit in New South Wales, Australia, has been smoldering since 4,000 B.C.
The characteristic temperature, spread rate and heat released during smoldering are low compared to flaming fires. Smoldering peat fires creep at a speed of 1 meter per day. Whereas flaming fires result in superficial heating of the soil, smoldering leads to sterilization and loss of mass above 90 percent (a layer of 5 meters is reduced to 30 centimeters). Moreover, these fires are difficult to detect with current remote sensing methods because the chemistry is significantly different, their thermal signature is much smaller, and the smoke plume is much less buoyant than the emissions from flaming fires.
Smoldering fires can be ignited by natural causes such as wildfires, lightning strikes, self-heating or anthropogenic factors, e.g., slash and burn, arson, mining activities or waste incineration. The most typical scenario for peat fires is when a fast flaming wildfire sweeps over a region burning the surface vegetation and igniting the peat if this is dry enough. The peat then smolders for a much longer time. This is what happens in Indonesia and probably what has occurred in Russia this summer.
Water content of the peat governs smoldering ignition. The depth and the area affected in case of fire are also be dictated by the water content of the peat layers. The maximum water content for boreal peat to ignite has been measured to be 55 percent by weight. Any water content bellow this means the peat can smolder.