Rescue crews have entered a West Virginia coal mine in search of the four miners missing since Monday’s explosion that killed twenty-five of their co-workers. The blast at Massey Energy’s Upper Big Branch coal mine was the worst US mining disaster in a quarter-century. Rescue efforts were delayed until dangerous gases could be filtered out of the mine. The prospect of finding survivors rests on hopes the missing miners escaped in time to airtight chambers with supplies of food and water.