From the text ... '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.'