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Seattle’s controversial waterfront tunnel is set to hit another milestone Tuesday, although the journey is far from over.

Nearly four years after Bertha the tunnel boring machine began digging, it is set to break through at Sixth Avenue North and Thomas Street in Seattle’s South Lake Union neighborhood.

Related: Watch Bertha break through

It will be still be two years before the tunnel opens, as crews need to finish double-decker lanes and other infrastructure.

How did we get here?

The tunnel is nine years in the making, as then Governor Christine Gregoire announced plans to tear down the Alaskan Way Viaduct in January 2008. A year later, the state, county, and city announced an agreement to replace the Viaduct with a tunnel.

By spring 2009, Gregoire approved a $2.8 billion funding plan. However, litigation is now underway to determine who will pay for the nearly $500 million in cost overruns.

Four years after the funding was approved, the world’s largest tunneling machine was shipped from a factory in Japan to Seattle. Crews assembled Bertha in three months, and she started digging July 30, 2013.

Project managers expected the nearly two-mile tunnel to be done in just over a year.

By December, Bertha was in trouble. The giant machine was overheating, and she dug in fits and starts as managers attempted to figure out what was wrong.

In January 2014, Bertha stopped and didn’t start digging for another two years. The problems were tied to seals and bearings.

Bertha resumed the dig in January 2016, but the problems continued. A sinkhole formed near the digging site, causing the project to be put on hold. Digging resumed after it was determined safe.

By April 2016, Bertha was back on track with just one more course correction in March 2017. Seattle Tunnel Partners discovered Bertha had veered about six inches off course. After adjustment, Bertha chugged along for the final 960 feet.

Timeline: The effort to dig the SR 99 tunnel