Deep Underground, Miles of Hidden Wildfires Rage - TIME

As far as 90 feet underground and 100 feet thick, miles of coal seams are continually burning in the U.S. and elsewhere, releasing toxic fumes that funnel up to the surface

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Spontaneous combustion of coal seams in the Bengiler coal mine in Turkey

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Spontaneous combustion of coal seams in the Bengiler coal mine in Turkey

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Coal-seam fire - Wikipedia