The coal industry in the United States has been in a long, steady decline for decades. But since 2012, with the availability of cheap natural gas and the ramping up of environmental regulations to control emissions from coal-fired power plants, that decline has become a full-scale collapse: coal-mining employment has shrunk from 89,800 to 55,500, a drop of 38 percent, according to the Bureau of Labor Statistics. And though the coal business is no stranger to boom-and-bust cycles, this is something different. Emissions limits are becoming ever tighter under regulations such as the federal Clean Power Plan, and the utility industry is  looking for more sustainable sources of power. Those jobs are never coming back.