The giant boring machine known as Bertha will begin digging the new State Route 99 tunnel under Seattle sometime Tuesday afternoon, the Washington State Department of Transportation said.

The tunnel will stretch from Pioneer Square to South Lake Union and carve an underground path that is scheduled to take 16 months to complete. The tunnel is scheduled to open to traffic in late 2015.

Total cost for the project is $3.1 billion.

WSDOT is placing more than 3,000 instruments on structures, creating a labyrinth of lasers and mirrors that can measure whether buildings are moved by the tunnel construction below.

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