The world's largest tunnel-boring machine is being assembled in Seattle. Called "Bertha," the borer will be used to dig the tunnel for the Highway 99 project along the city's waterfront. Once in place, the 300-foot-long borer will work in a "controlled environment" for the first 1,000 feet so observers can make sure all works as planned. "When the machine gets out there 2,000 feet, we're going to start building the interior structure," said Chris Dixon, the project manager. "And that's going to follow along behind the tunnel drive."