To build this dam, engineers first drowned a waterfall twice the size of Niagara. You could hear it roaring from 20 miles away. Then they made one of the world's biggest rivers move out of the way, just to clear room for the construction site.It's the Itaipu Dam, on the Paraná River where Brazil meets Paraguay. Work started in 1975, and the first job was the river itself. Crews spent three years carving a 1.2-mile channel through solid bedrock, 500 feet wide and 300 feet deep.They hauled away 50