The generator room. This is what pays the bills. The main purpose of Hoover Dam is to provide irrigation to surrounding farmland and a reliable source of water for cities throughout the Southwest including Los Angeles and San Diego. The sale of hydroelectric power has paid for the entire cost of construction of the dam and its operation for two thirds of a century. These generators crank out 4 billion kilowatt-hours of energy annually -- enough for 500,000 homes! You'd think with that much juice on tap, they wouldn't have to worry about turning the lights off, but I guess not...