"As a concept, hydraulic fracturing has changed very little since the first wells were drilled in the late 1940s. In practice, however, what most people now know as fracking has undergone a transformation.

Then, as now, a well is drilled into a shale rock formation, and then fluid is pumped in at high pressure, opening cracks that release oil, gas, or both.

The combination of this technique with horizontal drilling and other advances has brought both the boom and the controversy now associated with fracking."

Christina Nunez reports for National Geographic March 25, 2015. [2]