In the United States, coal companies are required by law to leave behind safety barriers (coal block at least 300 ft thick) separating the old mine works from current operations or a coal outcrop because of the potential of huge unexpected inflows of methane gas and water or blowouts. However, this can be difficult when the old mine works, usually belonging to another company, were inaccurately mapped and/or poorly documented. When reliable surveying is in doubt, a company's normal response will be expensive surface drilling to verify the old mine works shown on their maps. Drilling also will have a...

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