16 photos that show the dirty way the world gets an important energy source

While global coal use has fallen significantly in recent years, so far in 2016 it has seen a slight rebound, placing among the top five best performing commodities, according to Citibank.

And producers expect continued improvements this year. "Assessing the remainder of 2016, we are beginning to see some positive signs in the domestic thermal coal markets," said Joe Craft, the CEO of Alliance Resource Partners and Alliance Holdings GP.

Energy firm Cloud Peak noted in an earnings release that, "if summer burn is strong, utilities are expected to rebuild their stockpiles in anticipation of winter demand. This scenario creates the potential for strong shipments and increasing sales this fall."

Recent improvements don't augur a return to past highs - coal production has declined significantly in the US - but they do indicate that coal remains an in-demand fuel source around the world. And as long as that is the case, it will have to be pulled from the earth. As the photos below show, that can be a dirty process.