A thousand feet into the dig beneath Downtown Seattle, the tunnel boring machine known as Bertha is going nowhere.
Late Friday, as it crossed between Main and Jackson streets, engineers noticed resistance on the drill head.
Then it came to a stop when it hit some kind of obstruction.
"We're being really cautious. We want to make sure we don't damage this $80-million machine," said project spokeswoman KaDeena Yerkan.
The machine itself is not damaged.
Experts have spent three days so far trying to figure out exactly what Bertha struck in a layer of glacial till 60 feet beneath the surface.
It might be a big boulder, or something else.
Engineers are now considering drilling vertical holes ahead of the machine to feel out what's there.
Another option is to back the drill up 18 inches and send divers through the face of the drill into the water to look around.
But there's a risk that soil could cave in.
Once experts figure out what Bertha struck, the next decision will be what to do about it.
Project engineers hope to make some decisions in the next couple of days.
It is not yet clear what impact this will have on the project schedule.