Tunneling is hard work. Tunneling two miles through uneven material under a major city is nearly impossible. But that’s the task that faces “Bertha,” a 326-foot-long, rocket-shaped tunneling machine poised to dig a transportation tunnel under Seattle.

“Everybody’s watching Seattle, because this is the trickiest, most ambitious, most technically challenging operation now going on in the world,” said Douglas B. MacDonald, a former Washington state secretary of transportation, in an article in the Los Angeles Times about the project.

Bertha, which began digging in early August, weighs 7,000 tons and is as tall as a five-story building. It was built in Japan and came across the Pacific in 41 pieces. It was designed to handle the rough, uneven glacial fill found under Seattle, which includes clay, sand, silt, stone, and boulders.

Bertha Tunnel Digger

The two-mile tunnel Bertha is digging will replace an unsightly double-decker highway that was damaged during an earthquake in 2001. The highway currently runs along the waterfront, separating downtown Seattle from the charms of the coast.

After the 2001 earthquake, Seattle officials considered several options for replacing the 60-year-old highway – including a Boston-style “dig-and-cover” operation – but settled on tunneling as the least disruptive solution. When the tunnel is finished and the highway is torn down, the resulting new acreage will contain a city park and a surface street.

Bertha is starting slowly, at about six feet per day, so workers can make sure everything is working well. Eventually she will accelerate to 35 feet per day, and she should finish the whole dig by September 2014.

During the dig Bertha will pass under the highway, which is still in use, as well as under the foundations of 160 buildings, an old railway tunnel, a major sewer line, another automobile tunnel, and the foundation of Seattle’s monorail. At its lowest point the tunnel will be 200 feet below the surface.

Bertha will return to the surface near Sixth Avenue North and Harrison Street. She is laying the road surface behind her as she goes, but other necessary work will delay the opening of the tunnel until sometime in 2015.

To view photos and interesting graphics of Bertha and the path she is taking, click here.