The breakthrough of the cylindrical drilling machine marks a major step in one of the most ambitious American municipal projects in recent years. Once complete, 2 miles (3 km) of Highway 99, an elevated roadway along a densely populated waterfront, will be rerouted to run beneath the city of 650,000 people.
A sinkhole, a two-year delay and a $480 million claim by contractors have challenged the $3.1 billion project since it began in June 2013.
The underground highway, which had initially been slated to cost $2 billion and be completed by the end of 2015, has been widely compared with Boston's 16-year "Big Dig" tunnelling project, which suffered through cost overruns, design flaws, worker fatalities and other problems.
After emerging into a large open-air pit a few blocks from Seattle's Queen Anne neighbourhood, Bertha will be cut into pieces and hauled away over several months.
The 57-foot (17 m) wide borer made by Japan's Hitachi Zosen Corp. cost $80 million and was the largest in the world when tunnelling started in 2013.
While the project's $3.1 billion price tag is comparatively small - the Big Dig cost an estimated $22 billion - progress was complicated by plans to dig beneath some of the most tightly packed neighbourhoods in downtown Seattle.
Bertha overheated and stalled partway through the project in December 2013, putting completion into doubt. Tunnelling was delayed two years as engineers dug a 120-foot (37 m) access pit to make repairs.
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