It turns out that Bertha’s issues may run much deeper than steel—or a few, mind you—pipes clogging the cutter head. Even with steel and concrete out of the way following an eight-week shutdown of North America’s widest tunnel-boring machine, Bertha is back on the inactive list.

This time overheating is the cause. But the root is unknown.

Contractor Seattle Tunnel Partners shut down Berthan on Dec. 6 after it struggled to make headway in the 9,270-ft planned route just over 1,000 ft into the project. While removing water from the tunneling site and test boring to find obstructions, the Washington State Dept. of Transportation expected to find a large obstacle in Bertha’s path. Over an eight-week span, crews found only steel pipes and a concrete chunk.

With the cutter head cleared of debris, Bertha took a test move forward on Tuesday, Jan. 28. The machine went two feet, allowing crews to finish up a concrete ring that had been held in limbo from December. On Wednesday, Bertha moved another two feet, but temperatures 1.5 times the normal level, upward of 140 degrees at the head, forced Bertha back into standby.

The 1.7-mile tunnel construction started on July 30 and will eventually replace the aging Alaskan Way Viaduct. The $80 million machine was supposed to help get the tunnel open by the end of 2015, but that is predicated on completing tunneling by the end of this year. Now behind schedule and with another unplanned and unspecified wait on the way, expect litigation or intense negotiations to enter the project.

To move past the overheating, WSDOT has started a review—with tunneling experts—to find the cause of the heat near the cutter head. The good news? There hasn’t been the same heat spike near the machine’s motor. But everyone already knows the bad news and until the source of the issue is found, the entire $2 billion project with $120 million in contingency funds can’t move forward at its planned pace. We can blame Bertha, but we don’t know why.

Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He also writes for TIME, Popular Mechanics, Sports Illustrated and more. You can follow him on Twitter at @tdnewcomb or visit his website here.