"Today is the day," spokeswoman KaDeena Yerkan said. "Everyone here is very excited."
The tunnel will go under about 200 buildings, but officials don't expect any serious problems from the machine they call Bertha.
"We know exactly the path of Bertha and what she'll encounter along the way," Yerkan said.
The tunneling crew will be monitoring for settling and vibrations and is prepared to keep building foundations secure.
"So we're not worried buildings are going to tip," Yerkan said.
Transportation officials have set up Bertha with her own Twitter account to provide updates.
Bertha is 326 feet long and weighs 7,000 tons. It will leave a tunnel nearly 58 feet in diameter.
The $80 million machine is part of the $3.1 billion project to replace the Alaskan Way Viaduct, the double deck highway along the downtown Seattle waterfront. Built in 1953, it has carried 110,000 vehicles a day. Officials said the structure had to be replaced because it could collapse in an earthquake. Its removal is part of a project to renovate the waterfront, rebuilding the seawall, improving surface streets and adding new vistas of Elliott Bay.
Bertha was built in Japan and arrived by ship in April in 41 pieces. It was reassembled in a pit near the CenturyLink Field and Safeco Field stadiums.
It will take about 14 months to complete a nearly 2-mile tunnel. Bertha is projected to punch through to the surface near south Lake Union by October 2014. Traffic is expected to start using the four-lane toll tunnel by late 2015.
The Transportation Department and the contractor, Seattle Tunnel Partners, held a June 20 dedication ceremony with Washington Gov. Jay Inslee calling it an historic occasion.
Bertha is named for Bertha Knight Landes, the first and so far only woman to serve as mayor of Seattle. She was elected in 1926.