What are the so-​called “waters of the United States” and why did the Obama administration expand the definition? To understand that is to understand the evolution of environmental regulation in the United States and how the nation’s courts have interpreted what’s protected.

Determining where water begins and ends seems a matter of physical properties, of whether two hydrogen molecules are bonded to oxygen, and how much of it exists. Where is it located, and will people eventually drink it? Swim in it? Fish in it? This molecule, essential to life, has become another point of…

Orange plastic curtains protect passers-​by from blow torch-​wielding workers who are learning to weld in a new training facility for Steamfitters Local 449. The building on the border of Butler and Beaver counties represents an $18 million bet on a building boom that economists and government officials say the nearby Shell cracker plant…