The Gas Explosions Research Facility at Lake Lynn Experimental Mines was used to determine the detonability limit of methane for a 1-meter diameter tube as a function of the percent of methane in air. The measurements showed detonation limits of 5.3% (lean) and 15.6% (rich). A method for extrapolating these limits to larger systems, more relevant to coal mine tunnels, was proposed based on a simple scaling law and some empirical information on the number of cells required for a detonation to propagate in closed, open, and partially open geometries. The scaling law reproduces the measured detonation-cell sizes measured in the 1-m tube. Applying this to a tunnel the size of a coal mine produces a detonability limit less than the currently measured flammability limit for methane/air at atmospheric conditions, which raises interesting questions for detonation and combustion theory and suggests measurements in larger tubes.