On a scorching August day in southwestern Indiana, the giant Gibson generating station is running flat out. Its five 180-foot-high (54.9-meter-high) boilers are gulping 25 tons (22.7 metric tons) of coal each minute, sending thousand-degree steam blasting through turbines that churn out more than 3,000 megawatts of electric power, 50 percent more than Hoover Dam. The plant's cooling system is struggling to keep up, and in the control room warnings chirp as the exhaust temperature rises.

But there's no backing off on a day like this, with air conditioners humming across the Midwest and electricity demand close to record levels. Gibson, one of the biggest power plants in the country, is a mainstay of the region's electricity supply, pumping enough power