fires in the United States burning today, most are found in the state of Pennsylvania. Some fires along coal seams are natural occurrences. Some coals may self-ignite at temperatures as low as 40 °C (104 °F) for brown coal in the right conditions of moisture and grain size. The fire usually begins a foot or two inside the coal at a depth in which the permeability of the coal allows the inflow of air but in which the ventilation does not remove the heat which is generated. Self-ignition was a recognised problem in steamship times. One well known source of fires is mining breaking into a high pressure cavity of methane gas which on release can generate a spark of static electricity to ignite the gas and start a coal explosion and fire. The same gas static is well known in ships and care has to be taken to ensure no such static sparking can occur. Two basic factors determine whether spontaneous combustion occurs or not, the ambient temperature and the grain size: The higher the ambient temperature, the more quickly the oxidation reactions proceed. The grain size and structure determine its surface area. Kinetics will be limited by availability of reactant, which in this case is carbon exposed to oxygen. Wildfires (lightning-caused or others) can ignite the coal close to the surface or the entrance of a mine, and the smouldering fire can spread through the seam, creating subsidence that may open further seams to oxygen and spawn future wildfires when the fire breaks to the surface. Prehistoric clinker outcrops in the American West are the result of prehistoric coal fires that left a residue that resists erosion better than the matrix, leaving buttes and mesa. It is estimated that Australia's Burning Mountain, the oldest known coal fire, has burned for 6,000 years. Globally, thousands of inextinguishable mine fires are burning, especially in China where poverty, lack of government regulations and runaway development combine to create an environmental disaster. Modern strip mining exposes smouldering coal seams to the air, revitalizing the flames. Rural Chinese in coal-bearing regions often dig coal for household use, abandoning the pits when they become too deep, leaving highly combustible coal dust exposed to the air. Using satellite imagery to map China's coal fires resulted in the discovery of many previously unknown fires. The oldest coal fire in China is in Baijigou (白芨沟, in Dawukou