"Researchers who mapped methane concentrations on the streets of the nation's capital found natural gas leaks everywhere, at concentrations of up to 50 times the normal background levels. The leaking gas wastes resources, enhances ozone production, and exacerbates global warming—not to mention powering the city's infamous exploding manholes. Most of the natural gas we burn for heat and on stovetops in the United States is methane, a simple carbon atom surrounded by four hydrogens. Carbon dioxide gets more press, but methane is the more powerful agent of global warming, 21 times more effective than carbon dioxide at trapping heat in the atmosphere. And methane levels are rising fast. Methane levels in the atmosphere were just 650 parts per billion a century ago, versus 1800 ppb today."
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http://science.slashdot.org/story/13/03/26/2322207/washingtons-exploding-manholes-explained