Demand for natural gas has created a hydraulic fracturing or fracking boom; since 2008 over 5,000 new wells have been drilled nationwide. Workers at Chesapeake Energy, one of the biggest gas companies conducting fracking, are seen on the job site near Towanda, PA. David Gilkey/NPR hide caption

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Conservator Angelyn Bass cleans and stabilizes the surface of a wall of a Mayan house that dates to the ninth century. The figure of a man who may have been the town scribe appears on the wall to her left. Tyrone Turner/Copyright 2012 National Geographic hide caption

itoggle caption Tyrone Turner/Copyright 2012 National Geographic

Researchers say our brains are probably wired from an evolutionary sense to encourage running and high aerobic activities. Above, a man runs past the Sydney Harbour Bridge on April 22. Ryan Pierse/Getty Images hide caption

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A water truck heads up Colorado Road 215 along Parachute Creek. Water is key to extracting natural gas from deep underground. David Gilkey/NPR hide caption

itoggle caption David Gilkey/NPR

An artist's impression of a group of Yutyrannus. The 30-foot-long dinosaurs were covered with downy feathers — likely to keep the animals warm. Dr. Brian Choo/Nature hide caption

itoggle caption Dr. Brian Choo/Nature

A wobbling of the Earth on its axis about 20,000 years ago may have kicked off a beginning to the end of the last ice age. Glaciers in the Arctic and Greenland began to melt, which resulted in a warming of the Earth, a new study says. Above, Greenland's Russell Glacier, seen in 1990. Veronique Durruty/Gamma-Rapho/Getty Images hide caption

itoggle caption Veronique Durruty/Gamma-Rapho/Getty Images

This NASA map shows the size of aerosol particles in the atmosphere. Green areas indicate larger, more naturally occurring particles like dust. Red areas indicate smaller aerosol particles, which can come from fossil fuels and fires. Yellow areas indicate a mix of large and small particles. NASA Earth Observations hide caption

itoggle caption NASA Earth Observations

Towers carry electrical lines in San Francisco. The electricity grid is a web of power stations, transformers and transmission lines that span the continent. Justin Sullivan/Getty Images hide caption

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A liquefied natural gas tanker arrives at a gas storage station east of Tokyo on April 6, 2009. The shuttering of Japan's nuclear power plants has driven an increased reliance on natural gas and other fossil fuels. AFP/Getty Images hide caption

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Steam rises from the cooling towers of nuclear reactors at Georgia Power's Plant Vogtle in Waynesboro, Ga. The U.S. Nuclear Regulatory Commission approved Southern Co.'s application to begin full construction of the nation's first new nuclear units since 1978 at Plant Vogtle. Mary Ann Chastain/AP hide caption

itoggle caption Mary Ann Chastain/AP