When you have rotting material or chemical soak material buried under the dirt it generates heat and can combust.With the landfill in our area they use foam pieces from an autoparts supplier and this stuff and all the other garbage can smolder under there for days if not detected  until it breaks through to the surface.This is very tough toput out because it's like a bog fire it could have a large area underground that can't be seen that is now on fire and will spread even larger having oxygen feeding it now due to it breeching the surface.