The machine called Bertha ran into something last week and was shut down Saturday about 1,000 feet from the start. The $80 million machine is designed to break up boulders, so there's speculation about what it hit.
Engineers are considering drilling down 60 feet to the object as one of the ways to break up or remove the obstruction. A large crane equipped with a drill bit was brought to the site Wednesday morning.
Experts estimate it will take until Friday to identify the obstruction.
The nearly two-mile tunnel is supposed to be completed by the end of 2015, creating a four-lane replacement for the Alaskan Way Viaduct on Highway 99.
But already it has experienced at least three delays - one caused by union picketing over work assignments, another by a sinkhole near Jackson Street and now the mystery object.
WSDOT says the latest trouble started Friday when Bertha's five-story tall cutter head felt some resistance, then stopped. WSDOT says engineers with Seattle Tunnel Partners, the company in charge of building the viaduct replacement tunnel, have been consulting with other experts to identify the obstruction - whether it's natural or manmade.
They say Bertha wasn't damaged in any way. They're keeping her idle until they decide whether crews need to dig the obstruction out from above or if Bertha can charge through it.
Bertha has dug 1,000 feet of tunnel since July. She's sitting 60-feet underground between South Jackson Street and South Main Street among a mix of native dirt and fill tossed into place from as early as the 1800s.
She has just 450 more feet to travel before leaving that fill behind. It will also mark the end of phase one in the $4 billion tunnel project that will stretch 1.7 miles from adjacent to Safeco Field to Battery Street.
WSDOT doesn't know how long Bertha will remain stuck, but they say it's too early to say whether the delay will affect the project's bottom line or it's scheduled opening in late 2015.