A sheetlike accumulation of combustible gases under the roof of a mine roadway where the ventilation is too sluggish to dilute and remove the gas. Although the term is new, the hazard existed since the earliest days of coal mining. A combustible gases layer may be specified as one in which the gas is 5% or over and of a length greater than the width of the road in which it occurs. See: pocket of gas See also: stratification of methane