On average, there were 633 sanitary landfills in operation in 1990–92 in Finland. Annually, 380 landfill fires occurred, one-quarter of which were deep fires. The total amount of waste burnt in landfill fires was estimated to be 84,000 tonnes year−1. Unsufficient covering and compacting, ash disposal and deliberate fire starting were reported to be the most typical reasons for waste ignition. The most severe deep fires lasted for 2 months. A 10 m high, 35,000 m3 large experimental bank of waste was built, instrumented and set on fire from a 7 m deep well located at the centre of the area. Wood was burnt in this ignition well to produce glowing embers, which were covered with waste and soil layers. No spread of fire was detected after covering the ignition well. However, the fire was not extinguished by covering but remained smouldering throughout the experiment. To extinguish the fire, it was necessary to dig up the smouldering fire and cool it.