Coal dust sparked an invisible fire that heated the soil to 800 degrees and burned the feet of an 8-year-old boy, firefighters said.
Matt Bershinski walked into the fire Monday as he was playing in a park. It melted a plastic clogs to one of his feet and left him with second-degree burns.
There were no smoke or flames, which led firefighters to first suspect an underground mine was burning. Mining experts and geologists ruled that out but found a 2-foot-deep layer of coal dust
Hot weather and sunshine may have ignited the dust, authorities said.
Al Amundson of the state Division of Reclamation, Mining and Safety said coal is constantly oxidizing and can ignite if the heat from oxidization is not carried away by wind.
Officials believe the dust was dumped by workers from a mine that closed in 1957.