What is unusual about the massive piece of clinker in front of you?

Fires need oxygen, even when they are burning underground. As the coal fire burned deep into the hillside, cracks in the rock layers allowed air to be sucked down into the fire. Fire burned up the cracks and baked the rocks nearby, forming vertical "chimneys." Chimneys are the hottest part of the coal fire and bake the rock inside into a very hard clinker called porcellanite which is especially resistant to erosion.

This chimney you are looking at was exposed when softer sediments around it eroded away. 

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There are many signs that large coal vein fires have burned throughout the park in the past. Even today, coal fires can sometimes be found shaping and changing the landscape of the badlands.

Geology is not only a study of the past; it is an ongoing process.