From Australia and China to South Africa and Eastern Europe, the global economy is being transformed by the extraction of huge amounts of natural gas from shale rock. The United States has played a major part in this revolution; new “plays,” as fields of shale gas are known, are now producing in Texas, Louisiana, Illinois, Arkansas, Colorado, West Virginia, and other states. In the last three years, more than 3,000 gas wells have been drilled in western Pennsylvania’s share of the huge Marcellus shale formation. With more and more producers in the business, the price of natural gas has dropped...