The Gleision Colliery mining accident was a mining accident which occurred on 15 September 2011 at the Gleision Colliery, a drift mine at Cilybebyll in Neath Port Talbot, in Wales. The accident occurred while seven miners were working with explosives on a narrow coal seam. Following a blasting operation into a separate disused flooded mine network to increase air-circulation, the tunnel in which the miners were working began to fill with water. Three of the miners escaped, with one being taken to hospital with life-threatening injuries, while the others were trapped underground. A search and rescue operation was launched to locate the four remaining miners, but they were found deceased the following day. The incident is the worst mining disaster to occur in Wales for three decades.