National Public Radio: If you've flown across Nebraska, Kansas or western Texas on a clear day, you've seen them: geometrically arranged circles of green and brown on the landscape, typically half a mile in diameter. They're the result of pivot irrigation, in which long pipes-on-wheels rotate slowly around a central point, spreading water across corn fields.
Yet most of those fields are doomed. The water that nourishes them eventually will run low.
That water comes from a huge pool of underground water known as the...