An underground fire is a fire that burns beneath the surface of the ground, often in coal seams or other combustible materials. These fires can occur naturally or be ignited by human activities, such as mining or discarded cigarettes. They can smolder for years, producing smoke and harmful gases that can escape to the surface. One of the most well-known examples of an underground fire is the Centralia fire in Pennsylvania, which has been burning since 1962. Underground fires can be difficult to extinguish and may require extensive efforts to control, posing risks to the environment and nearby communities.