The signs as you approach the Eastern Pennsylvania town of Centralia ominously warn of danger and death. Ignore those signs and you are soon surrounded by a post-apocalyptic scene. The houses are razed, the fields empty, and the roads mangled by overgrowth and vegetation. And then there's the toxic smoke, billowing from beneath the mostly abandoned town through fissures and holes in the earth. The ground below is literally on fire and it has been for more than fifty years.

A seam of coal thirteen kilometers long and encompassing more than fifteen square kilometers is burning under the old mining town. This blaze started in 1962, most likely when sanitation workers began burning trash at a landfill too close to the entrance of an abandoned mine. Today it remains the most infamous coal fire in the United States.

But the phenomenon is hardly unique. More than 150 coal fires are burning right now in the U.S., while thousands more rage in at least 20 other countries around the world. In China alone, underground coal fires are thought to burn more than 200 million metric tons of coal each year, roughly 10% of the nation's total annual production. And it's not just careless humans who are to blame: coal fires can, and often do, start spontaneously. And by spontaneously here, I don't mean like "suddenly" or "quickly." When we're talking about thermodynamics, a reaction is spontaneous when it occurs without the input of energy from an outside source to drive it along, it just goes.

Low grade, soft coal is more likely than other types to spontaneously combust, sometimes at temperatures as low as 40 degrees Celsius, especially when it's near the surface and comes into contact with open air.

It begins when the coal reacts with oxygen in the air, or oxidizes, the same way that iron exposed to air oxidizes and rusts. When things oxidize, they give off heat. Burning is just rapid oxidation giving off lots of heat. Eventually, the temperature of the coal gets high enough for long enough that it evaporates any remaining moisture in it. That's when ignition can occur, and when bad things start to happen.

Coal fires can be caused non-spontaneously like by a lightning strike or a brush fire in addition to human activity, but either way, once it's ignited, the fire takes advantage of the all-you-can-eat coal and oxygen buffet.

Many coal fires smolder for years unnoticeable in the ground, but eventually, the coal starts turning to ash. This creates voids in the earth that cause the ground above to collapse, creating sinkholes that can destroy roads and buildings. And these collapses just give the fire access to more oxygen, extending the life of the blaze.   

Coal seam fires move relatively slowly, maybe two meters per month. But they can easily spread to other nearby deposits. The fires also, of course, release a toxic brew of emissions, including benzene, sulfide, mercury and arsenic, not to mention methane and CO₂.

Coal fires are especially troublesome when they ignite in underground mines, where the oxygen is plentiful, thanks to ventilation systems and the blaze becomes nearly impossible to extinguish. This is the case in Centralia, where the fire continues to burn at depths of up to 100 meters. Officials tried for nearly two decades to put out the Centralia fire, using methods that included digging trenches and pouring down wet sand and gravel and cement in an attempt to cut off the oxygen supply. But the cement in particular tended to have the opposite effect, thanks to temperatures in excess of 537 degrees that burned away all the fill material, leaving more gaps for oxygen. Nothing worked - even fighting the toxic flames at the surface proved fruitless, and eventually the U.S. government allocated $42 million to relocate residents and businesses. What was once a town of more than 2,000 now has a population of a dozen or so holdouts who refuse to leave. The residents who left won't be returning: scientists estimate that there's enough coal beneath Centralia to fuel the fire for another 250 years. And yet, that's nothing compared to Australia's aptly-named Burning Mountain, under which a 6,000-year-old fire continues to burn through a seam of coal in the earth.

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