The PRB was the home of the top nine producing mines last year, but the number of PRB mines in the US top 25 declined compared to 20 years ago as large operations emerged in the east.

Of the current top 25 mines, two that did not produce any coal in 1994 — the earliest year for which SNL Energy data is available — were in the Illinois Basin, illustrating the emergence of high-sulfur coal from the region as more utilities installed environmental controls on their power plants.

“The top Illinois Basin mines are only growing,” according to SNL.

Alliance Resource Partners LP's River View mine in western Kentucky, which did not start producing coal until 2009, produced 9.3 million tons in 2013, ranking No. 14 on the list of highest-producing mines in the US.

According to early data released by the US Mine Safety and Health Administration, River View produced 2.4Mt in the first quarter of 2014, just shy of its all-time quarterly record.

Peabody Energy Corp's Bear Run mine in Indiana, which started producing coal in 2010, produced 8.3Mt in 2013, ranking No. 20 on the list.

According to early data released by MSHA, Bear Run produced 2.1Mt in the first quarter of 2014, its highest-ever first-quarter production total.