A landfill fire occurs when waste

Waste

Waste is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea, sweat or feces. Litter is waste which has been disposed of improperly...

 disposed of in a landfill

Landfill

A landfill site , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment...

 ignites and spreads. In landfills that do not cover their waste with daily cover

Daily cover

Daily cover is the name given to the layer of compressed soil or earth which is laid on top of a day's deposition of waste on an operational landfill site. The cover helps prevent the interaction between the waste and the air, reducing odors and enabling a firm base upon which for vehicles to operate...

, biological decomposition

Decomposition

Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...

 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 intrution control is the best method to prevent and fight subsurface landfill fires. "Fuel quenching", by allowing landfill gas

Landfill gas

Landfill gas is a complex mix of different gases created by the action of microorganisms within a landfill.-Production:Landfill gas production results from chemical reactions and microbes acting upon the waste as the putrescible materials begins to break down in the landfill...

 build-up, can work well, especially in conjunction with maintenance of the daily cover

Daily cover

Daily cover is the name given to the layer of compressed soil or earth which is laid on top of a day's deposition of waste on an operational landfill site. The cover helps prevent the interaction between the waste and the air, reducing odors and enabling a firm base upon which for vehicles to operate...

 of soil or material places on landfills.

Nearby streams can be threatened by leachate

Leachate

Leachate is any liquid that, in passing through matter, extracts solutes, suspended solids or any other component of the material through which it has passed....

 pools which may form if water is used to extinguish fires in landfills. There is also the danger that the landfill's membrane

Landfill liner

A landfill liner, or composite landfill liner, is intended to be a low permeable barrier, which is laid down under engineered landfill sites...

, 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.

Notable landfill fires

• On January 26, 1998, in Maalaea, Hawaii
  Maalaea, Hawaii
  Māalaea is a census-designated place in Maui County, Hawaii, United States. The population was 454 at the 2000 census. Māalaea is the site of a small boat harbor, as well as the state-of-the-art Maui Ocean Center.-Geography:...
  
  , a fire 15 to 20 feet underground. The fire was eventually deemed to be extinguished in a matter of weeks, with injections of more than 1,000 pounds of liquid carbon dioxide. It continued to smolder for 4 months.

• An underground landfill fire that was discovered in December 1996 in Danbury, Connecticut
  Danbury, Connecticut
  Danbury is a city in northern Fairfield County, Connecticut, United States. It had population at the 2010 census of 80,893. Danbury is the fourth largest city in Fairfield County and is the seventh largest city in Connecticut....
  
   caused a strong odor like rotten eggs due to the high concentration of hydrogen sulfide
  Hydrogen sulfide
  Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
  
  . The fire lasted for weeks and the town was forced to install a gas recovery system, the cost of which exceeded $1 million.

• In early November 1999, at the Delta Shake and Shingle Landfill in North Delta, British Columbia
  North Delta, British Columbia
  North Delta is a largely middle-class bedroom community situated in the Lower Mainland, in British Columbia, Canada. The community is the most populous of the three communities that make up the Corporation of Delta. North Delta is home to numerous parks and recreational opportunities...
  
  . the fire burned between 20 and 30 metres (about 100 feet) deep. On November 27, Delta's Mayor declared a state of local emergency. Extinguishing the fire took slightly more than two months and cost more than $4 million (Canadian).

• On September 2, 2007 a large fire at the Fredericton Regional Landfill forced residents to stay indoors because of fears the smoke could be toxic.http://www.cbc.ca/canada/new-brunswick/story/2007/09/02/landfill-fire.html