Contrary to popular belief, underground fires are a surprisingly frequent phenomenon, the fuel being coal and the fire travelling along the seams, or the thin layers of rock or mineral. Such fires travel slowly due to the limited supply of oxygen, but can burn for a very long time: the underground fire at Burning Mountain Nature Reserve in Australia is thought to have been continuing for the past 5,500 years. The number of such subterranean fires worldwide is countless. According to one study, subterranean fires in China alone are consuming some 200 million tonnes of coal a year and pumping into the air as many pollutants as all the cars in the United States. Along with numerous human-related factors, such fires are also contributing substantially to global warming.