What's blocking Bertha? Perhaps a 'glacial erratic'...

SEATTLE -- It’s been almost a month since the world’s largest tunneling machine along Seattle's waterfront came to a screeching halt, stopping work on the tunnel to replace the Alaskan Way Viaduct.

The Washington State Department of Transportation was hoping to get deep enough Thursday to figure out what is exactly blocking Bertha.

There is a lot of speculation, but David Williams, a writer specializing in urban geology who is studying the area where Bertha is stuck, said it could be similar to the landmark found in Seattle's Wedgwood neighborhood known simply as the Big Rock.

“Well, it’s very unusual,” resident Gary DeGorgue said of the Big Rock.

Sitting at the corner of 28th Avenue NE and NE 72nd Street,  the rock is more than 19 feet tall.

“Awesome, I really think it is,” resident Carol Cassinelli said.

But something similar could be the reason why Bertha, a 7,000-ton machine, is at a standstill 60 feet below the surface miles away.

“From a geological point of view, that’s the best guess,” Williams said.

“When the first Europeans arrived, where Bertha is right now was water,” Williams said.

From the Cascades to the Olympics, the landscape was covered with ice more than 16,000 years ago. When the glaciers started moving, they carried rocks along the way.

“Those could be any size. As you can imagine,you have a 3,000-foot thick massive ice, it can push anything in its path,” Williams said.

That’s why geologists call the boulders a glacial erratic.

“A glacial erratic is a rock that’s pushed from point A to point B. As the glacier pulls back or retreats, it leaves behind the rocks, the most famous one is Plymouth Rock in Massachusetts,” Williams said.

WSDOT has 10 de-watering wells, pumping out water to lower the pressure so divers can see for themselves. For now, all experts can do is look to history for an answer.

WSDOT did not share how far they got on Thursday but they are holding a press conference on Friday at 1 p.m to update the public.