In this March 25, 2014 photo, a worker adjusts hoses during a hydraulic fracturing operation at an Encana Corp. gas well, near Mead, Colo. The first experimental use of hydraulic fracturing was in 1947, and more than 1 million U.S. oil and gas wells have been fracked since, according to the American Petroleum Institute. The National Petroleum Council estimates that up to 80 percent of natural gas wells drilled in the next decade will require hydraulic fracturing. (AP Photo/Brennan Linsley)

Photos: A look at fracking

In this photo essay, AP Photographer Brennan Linsley looks inside a walled-off fracking facility.

In this March 25, 2014 photo, a machine mixes sand and water, left, before it is pumped underground during a hydraulic fracturing operation at an Encana Corp. well pad near Mead, Colo. (AP Photo/Brennan Linsley)

A close-up look at fracking

Workers bustle at an oil and gas drilling site near Mead, Colo., a town of about 3,800 people north of Denver.