At approx 5:55 PM an inundation of methane occurred at the tailgate of the longwall face and at the MPL's in the back of the panel, in the panel #20 longwall section as a result of a cave in the gob. The mine was evacuated and MSHA notified when methane concentrations reached 4.98% at the MPL's. Ventilation was not interrupted and the methane began to go down almost immediately.