SAULT STE. MARIE, Mich. (WOOD) — They call it the “Achilles heel” of the North American economy: the Soo Locks in Sault Ste. Marie.

Most of the nation’s iron ore passes through the locks, which allow ships to negotiate the 21-foot drop along the St. Marys River between Lakes Superior and Huron. Some of the moving parts are a century old.

A recently released federal study warns that if the locks were to fail, it would cripple the economy — especially the automotive industry.

Most of the iron ore, which is used to make steel, goes through the Poe Lock. Owned by the U.S. Army Corps of Engineers and last rebuilt in 1968, it’s the only one big enough for the 1,000-foot lake freighters — more commonly known as lakers — that carry the ore from Lake Superior to steel mills all over the Great Lakes region.

If Poe fails, the impact would be first felt about 180 miles west of Sault Ste. Marie in the mining towns around Ishpeming.

“Everything that comes out of here has to go through the Soo. If it gets transported by ships, it has to go through the Soo,” explained Tony Slawinksi, who runs Handy Grocery out of the front of his home in Palmer, across the street from the mile-wide Empire iron ore mine.

“So yeah, if you shut that down, it would have a trickle effect and shut a lot of things down,” Slawinkski continued.

There are two iron ore mines in the UP, both within a few miles of each other. One of those, Empire, is shutting down soon, putting nearly 400 high-paid miners out of work. The rest of the ore comes from mines in Minnesota.

“If they can’t get their steel from here, then where else are they going to get it from? Because up here and in Minnesota, that’s where the iron mines are,” Slawinski said.

Mining would take the first hit, but that would be just the beginning.

The Poe Lock has never failed, but the U.S. Department of Homeland Security says if it did and remained closed for six months, the U.S. would fall into a deep recession.

Steel mills in Michigan, Indiana, Kentucky, Ohio, Ontario and Pennsylvania would shut down. Almost all automobile production in North America would stop. Nearly 11 million people would end up out of work in the U.S. and millions more in Canada and Mexico. Michigan would get hit hardest, the study says, with an unemployment rate of nearly 23 percent.

>>PDF: Report from the Department of Homeland Security

“There is no alternative. We don’t have rail lines that can take the massive material that is coming through these locks each and every day. We don’t have the trucks that can do it,” U.S. Sen. Gary Peters, D-Bloomfield Hills, said.

Thursday, Peters toured the locks to see firsthand what could go wrong.

“This is about the steel industry, it’s about energy, it’s about all the ripple effects,” he said. “It’s a situation that would impact every part of the state of Michigan.”

“If they don’t make steel, we don’t make cars, we don’t make trucks, we don’t make all the products of modern life,” he continued.

While it’s critical to maintain the locks, Peters said Congress will need to come up with the money to build a second large one.

“The plan is to take two of these smaller locks and basically make another large Poe Lock,” he explained.

That, he says, would cost at least half a billion dollars.

“It’s not a matter of if this lock will need to be shut down during the summer, it’s just a question of when and that’s why urgency to get this fixed is there,” Peters said.

Even if the money is approved, it would take six to 10 years to build a second large lock.

>>Video of Peters touring the Soo Locks: Part 1 | Part 2 | Part 3 | Part 4