In the report, the agency said employment was at a high of 92,000 workers in 2011. In 2018, the total was just 54,000. The hardest-hit region was Appalachia, which includes Pennsylvania, West Virginia, Ohio and part of Virginia.

Interestingly, production of coal peaked in 2008, according to the EIA; it wasn't until three years later that employment numbers hit their high. For scope, the US produced 1.2 billion short tons (1.08 billion tonnes) in 2008 from 1,458 mines but last year produced 756 million short tons (685.8 million tonnes) from 679 mines.

"As was the case with employment, much of coal's production decline was concentrated in the Appalachian region," researchers said. "More than half of the region's mines have closed since 2008, and production has fallen from 390 million [short] tons [353Mt] in 2008 to 200 million [short] tons [181Mt] in 2018."

While Appalachian mines are generally smaller than their Interior and Western US counterparts, the EIA did note a brief spike from 2016 to 2018 as jobs grew along with export tonnage demand.

At the same time, the agency confirmed that the decline in operating mines was much greater in scope than the noted changes in production and employment. That is generally because smaller mines have a relatively harder time surviving in a down market and are often the first to be mothballed.

The plus to that significant minus, according to the EIA's data, is that tonnage per labour hour grew gradually as smaller and less productive operations closed; in fact, 5.2 short tons (4.7t) per labour hour in 2011 rose to 6.2 short tons (5.6t) per labour hour in 2018.

"The large surface mines in the Powder River Basin (PRB) in Wyoming and Montana have much higher productivity, but even PRB productivity has declined as the region's producing coal seams become deeper and the amount of overburden, or top soil and rock above the coal seam, increases," the researchers said.

"In contrast, the Appalachia and Interior regions both have shown improvements in labour productivity between 2011 and 2018, largely because they are increasingly relying on less labour-intensive longwall and highwall mining systems and closing or idling the least productive mines."