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When Bertha, the world’s largest tunneling machine, got stuck a few weeks ago, a fifth-grade teacher at Seattle’s Thornton Creek Elementary seized the chance for his students to play history detective.

“They had just learned about the Great Seattle Fire when Bertha got stuck under the Seattle Waterfront,” said Todd Bohannon, and suddenly they had a mystery that made their study of state history immediately relevant:

What was blocking Bertha’s path?

Progress on the four-lane tunnel from Sodo to South Lake Union has stalled since Dec. 6. Bohannon assigned the class to write essays on what could be so large that the 54.3-foot-diameter drill couldn’t chew its way through it — a question that’s still confounding project engineers.

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“The possibilities are endless, but we had to narrow it down,” Bohannon said.

Of course the first thing the fifth-graders imagined was a buried flying saucer — a theory the teacher quickly nixed.

“The requirement was really for them to come up with a theory that was realistic and then for them to find an article that supported that theory,” he said.

Fifth-grader Marina Sanchez, writing that she felt “a bit like Nancy Drew,” captured the spirit of the essay like this:

People are buzzing about what could be beneath our very own city … monster, a haunted boat, a train car. It could be many things …

One of her theories is that a gale in 1882, which blew several rail cars into Elliott Bay, left one buried in Bertha’s path.

I think that when the railroad cars went flying, that one of the cars sailed onto the tidal flats and got buried in sawmill dust.

Classmate Zoe Kackman thinks it could be a rail car, too, but from a different event: the Great Seattle Fire of 1889 that left more than 25 blocks of downtown Seattle in smoldering ruins.

Based on what I’ve learned and read I think a train car and /or garbage fill is blocking Big Ber

Rachel Schmidt thinks the heat of the fire might have melted railroad tracks into an impenetrable blob.

In the aftermath of the fire the street level was raised 22 feet. My theory is that the melted tracks could have been used as fill for the ground.

Alex Thornewell had the same idea:

My theory is that some of the tracks by Jackson and Main street could have melted in the intense heat of the Great Seattle fire. After the fire the tracks became nonusable and the Seattleites buried the tracks.

The waterfront was home to several lumber mills and some students speculated that perhaps a huge buried sawmill blade is giving Bertha fits, Bohannon said.

Others wondered whether it could be a boat.

Another fifth-grader, Paden J. Grey, referenced a 1918 wreck that occurred around midnight when the A.J. Fuller — a ship that had been turned into a floating cannery anchored just offshore — was struck by a steamer called Mexico Maru and sank within 10 minutes.

Since Bertha is along the waterfront and Seattle has a deep water port, I was thinking that there are parts of a steel lined boat blocking Bertha from continuing on her path. A boat named the Fuller sank in front of Seattle caused by crashing into a boat named the Mexican Maru, leaving a ten foot hole in the bow of the ship.

George Fulton’s theory is less dramatic. He thinks it’s a big rock.

Bertha is made to drill through small boulders, brick and even something as dense as solid concrete. It is my belief that Bertha is sitting behind a glacial erratic, a big boulder deposited by a glacier around fifteen thousand years ago.

That also happened to be the initial guess of the tunnel-project leaders, but as of Monday, a half dozen soil probes in front of Bertha hadn’t bumped into anything big, leaving the mystery unsolved.

Bohannon said kids are as fascinated by Bertha itself as they are in what may be blocking it.

“So now there’s, like, five different pictures of Bertha in the classroom from different angles because they’re a little bit obsessed with what this machine is,” Bohannon said.

Thornton Creek Elementary’s emphasis on using extended projects to cover subjects the state requires, such as state history in fourth and fifth grades, gives teachers more flexibility to respond to current events than following a textbook.

So when Bohannon saw how interested his students were about Bertha, he quickly harnessed that enthusiasm.

“They’re engaged because it’s something that’s happening today, it’s not something that happened 150 years ago,” Bohannon said.

John Higgins: 206-464-3145 or On Twitter @jhigginsST

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