Seattle's Alaskan Way Viaduct, which runs along the city's waterfront near downtown, is a double-decker highway slated to be replaced with a tunnel. (Photos: FlickrCC/WSDOT)
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The World's Largest Tunneling Machine

The 7,000-ton machine, named after Seattle's first female mayor, is digging a tunnel this summer to replace the city's iconic highway, the Alaskan Way Viaduct.