A landfill fire occurs when waste disposed of in a landfill ignites and spreads. In landfills that do not cover their waste with daily cover, biological decomposition creates substantial heat and can cause material in the landfills to spontaneously combust. In the U.S. 40% of landfill fires are attributed to arson.
Landfill fires are especially dangerous as they can emit dangerous fumes from the combustion of the wide range of materials contained within the landfill. Subsurface landfill fires also, unlike a typical fire, can not be put out with water. They are similar to coal seam fires and peat fires. Oxygen intrusion control is the best method to prevent and fight subsurface landfill fires. "Fuel quenching", by allowing landfill gas build-up, can work well, especially in conjunction with maintenance of the daily cover of soil or material places on landfills.
Nearby streams can be threatened by leachate pools which may form if water is used to extinguish fires in landfills. There is also the danger that the landfill's membrane, a barrier placed under most modern landfills to prevent contamination of the underlying ground, will be destroyed or penetrated by the fire itself. Normally this liner prevents harmful liquids contained within the landfill from escaping into the groundwater and nearby streams. Destruction of the liner therefore leads to serious environmental problems.