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" It will take days and maybe weeks to know exactly what caused the explosion on Monday afternoon in Massey Energy&amp;#039;s Upper Big Branch mine, in Montcoal, West Virginia. With 25 miners dead, two hospitalized and four missing, it is the country&amp;#039;s deadliest mining accident in 25 years. Like other longwall mines, Upper Big Branch would have been a vast, efficient and dangerous place.  More than half of the nation&amp;#039;s electricity comes from coal, and 31 percent of that is mined from underground. Room-and-pillar mining, in which coal is excavated around a series of columns, dates back centuries to the exclusive use of picks and shovels. Modern longwall mining was introduced to the U.S. in the 1950s, and now more than 100 such mines, mostly in central Appalachia, produce the lion&amp;#039;s share of underground coal.  From an elevator-like entrance shaft deep underground, continuous miners\u2014cutting machines on wheels\u2014bore passages on both sides of seams of coal up to a quarter mile wide and a mile or more long. At the mine face, a massive shearer on self-advancing ceiling supports known as &amp;#039;shields&amp;#039; slides back and forth across the face like a giant cheese grater. Water sprays constantly against the coal face to dampen coal dust. After each pass, the whole apparatus, as wide as 1600 feet, lurches forward, letting the area behind the shields collapse. A conveyor belt catches the coal, moves it to another belt running along the side passages, and takes it to the surface, often several miles away. When a panel of coal is mined out, the longwall machine is moved to the next one. Over time, mines become enormous labyrinths of passages, and it can take miners a half hour or more to travel miles to the mine face in low-slung vehicles called mantrips.  To experience an underground longwall mine is to venture into a hazardous underworld. The mine passages are unlit, and so dark you can&amp;#039;t see your own hands. Water drips constantly, and sometimes pours, from above. Passages wind and plunge; they can be dead silent one moment, roaring with machinery the next. Longwall equipment, which runs 24 hours a day, seven days a week, is highly efficient. Even a large mine requires few miners, who carry emergency breathing apparatus and gas detectors at all times; the only light comes from their headlamps.  Dangers are everywhere: from falling roof or wall chunks; from working in tight confines with huge machines; and, most of all these days, from methane released from the excavated coal\u2014and the mines of central Appalachia are highly gassy. Methane is odorless, tasteless, invisible and, when mixed with air, highly explosive in volumes of five to 15 percent. &amp;quot;If you had five percent methane in your office and you lit a match,&amp;quot; says Christopher Bise, professor and chair of mining engineering at West Virginia University, and an expert in mine safety, &amp;quot;it would explode.&amp;quot;  Coal itself is a fuel, the finer the particle the more easily ignited. And should the volatile mix of coal dust and methane ignite, it produces deadly carbon monoxide (which is what happened at the Upper Big Branch mine on Monday: rescuers had to first punch holes into the mine to release the carbon monoxide before entering). Huge fans circulate fresh air through the mine and across the face\u2014you feel a constant wind inside\u2014and miners and machinery are equipped with gas meters; federal regulations limit methane/air concentrations to one percent, and if the gas exceeds that percentage equipment should automatically stop.  The mystery of the Upper Big Branch mine explosion, says Bise, will center on whether everything was working and there was simply a sudden and dramatic increase in methane, or whether the mine&amp;#039;s gas levels were systematically higher than regulations permitted. &amp;quot;To get enough methane in the workplace so quickly for such a large explosion to occur,&amp;quot; he says, &amp;quot;seems highly unusual. Over the past 30 years the number of major mine explosions, well, you can count them on one hand.&amp;quot; The explosion at the Sago Mine in 2006 that killed 12 was attributed to a freak lightning strike traveling through the earth into the mine, a scenario that Bise finds suspect. Every miner carries an oxygen mask and lining the tunnels leading to the mine face are indentations and sealable rooms stocked with food and additional breathing aids. Electricity is carried through wires along the walls, as are wires for communication. In the best possible scenario, miners would have time to don their masks and make their way to one of those rooms; in reality, explosions happen suddenly and can knock out all power and communications, leaving any miners who manage to survive the initial explosion in a cold, dark netherworld with limited breathing time.  There is a lot we still don&amp;#039;t know about what happened on Monday, except this: The explosion was sudden and so powerful it twisted the steel rails carrying the mantrip. And, cruelly, it happened at shift change, when the maximum number of miners was entering and exiting. &amp;quot;I hope,&amp;quot; says Bise, &amp;quot;that no one was violating the law and operating without proper ventilation, but that remains to be seen.&amp;quot; Carl Hoffman reported from a longwall mine when he wrote about coal mining for Popular Mechanics&amp;#039; August 2007 issue. He recently authored The Lunatic Express. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/science/energy/a5527/dangers-in-longwall-coal-mining/>