This month, the Department of Energy announced that a tank at the Hanford Nuclear Reservation in Washington state is leaking up to 300 gallons of radioactive waste a year. Nuclear sludge left over from Cold War plutonium production is drip-drip-dripping into American soil, infiltrating the groundwater, slowly making its way into our rivers. The leak is just another in a long line of mild disasters at America's most contaminated nuclear-waste site, a radioactive drop in the already-polluted Columbia River. Hanford is the worst kind of mess: the kind that humanity is capable of making, but not capable of cleaning up. It was the home of the world's first full-scale plutonium reactor and the epicenter of American nuclear production during the Cold War. Now the 586-square-mile campus is the subject of the largest environmental cleanup operation the United States government has ever undertaken. There are other sites in America with long nuclear histories-places like Oak Ridge, Los Alamos, Yucca Mountain. But none have become sprawling disasters with quite as much panache as Hanford. The human and environmental consequences of Hanford have spread beyond those borders, across Washington and Oregon. A decade ago a rash of radioactive tumbleweeds blew across the nearby plains. In the early 1960s, an irradiated whale was killed off the Oregon coast, having apparently been contaminated by nuclear waste flowing down the Columbia River.