Firedamp is flammable gas found in coal mines. It is the name given to a number of flammable gases, especially methane. It is particularly found in areas where the coal is bituminous. The gas accumulates in pockets in the coal and adjacent strata, and when they are penetrated, the release can trigger explosions. Historically, if such a pocket was highly pressurized, it was termed a "bag of foulness".