Wildfires (lightning-caused or others) can ignite the coal closer to the surface or entrance, 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.