According to the Associated Press, the blast occurred at 11.11am local time, Tuesday in the vicinity of Cloud Peak Energy’s Cordero Rojo mine.

Some in Gillette, some 16 miles away, reported feeling the earth shake. The US Geological Survey’s seismographs recorded the explosion to be as large as an earthquake measuring 4.5 on the Richter scale.

While blasting explosions are indeed a frequent occurrence in surface operations, it is far less common that they are recorded at such intensity.

Local law enforcement officials told the AP that no mine emergencies had been reported.

Cloud Peak officials did not release a public statement on any events at the complex.

The Cordero Rojo operation, which extracts from the Wyodak seam at a seam thickness range of 55 to 70 feet, mined and shipped about 39.5 million tons of low sulfur coal last year.

Wyoming is the top producing coal state in the US Powder River Basin mines in the state produced 426 million tons of coal in 2011.