The Duluth ship entry offers a sense of the scale of the problem. It's a narrow channel, one-third of a mile long, spanned by the aerial lift bridge. It's one of the most visited places along the Duluth harbor.
The walls on both sides of the channel are made of steel. Near the top, under a concrete cap, the walls are smooth, like steel siding. But down under the water, steel like this is getting rough and pitted.
There's a lot of steel in the Duluth-Superior harbor. Shipping docks are lined by similar plate steel. Even the very old docks made of huge timbers -- the timbers are held in place by steel pins.
And much of that steel is rusting away, a lot faster than it should.
A few chunks of that rusty steel can be found in Gene Clark's Superior office. He's an engineer with Wisconsin Sea Grant, a program dedicated to the sustainable use of the Great Lakes.
One piece of steel the size of a dinner plate tells the story. It came from nearby, in Superior. The side that had been up against the dirt for maybe 80 years is a little scarred, but fine. Clark flips it over to the side that had been exposed to the water.
"It's like someone took a small ice cream scoop and just scooped pieces of steel," Clark said. "And you just look at the thickness -- probably now just half thick."
The metal has become a rusty moonscape of craters. In some places in Duluth, the seawalls are rusted clear through, with holes big enough to stick your fist through.
Rust is a problem for ocean ports, but Clark said no one expected this in fresh water. And he said it's going to be expensive to fix.
"We're talking hundreds of millions of dollars just in the Duluth-Superior harbor itself, to totally replace all the docks that we're seeing damaged," Clark said.
Jim Sharrow has a few rusty props too, in his office at the Duluth Seaway Port Authority. One of the mysteries about the rust is where it shows up, and where it doesn't.
Sharrow said in Lake Superior, just outside the harbor, there is little to no rust, but it turns up inside the harbor.
"There's certain parts of the harbor where it's occurring at almost twice the rate as it is in other parts of the harbor," Sharrow said.
There's a long list of suspects -- from tannin-rich water, to stray electrical current, to chemicals like road salt. And, Sharrow said, it looks like the corrosion wasn't an issue until about 30 years ago.
"All along to us, it has appeared that something changed in our harbor in the 1970s to greatly increase the rate of corrosion," said Sharrow.
Recently they've observed similar rust in other places around Lake Superior far from Duluth.
"At places like Thunder Bay (Ontario), Two Harbors, along the South Shore, even in Bayfield and La Point Wisconsin. Even at a small DNR boat ramp in the northern Keweenaw (Michigan) Peninsula," Sharrow said.
But it's not happening everywhere. A 50-year-old ore dock in Silver Bay is virtually rust-free.
One theory for the rusting points to harbors with water that's rich in tannin, possibly fed from biologically active places like bogs.
One promising lead points at tiny living creatures. Dr. Randall Hicks studies aquatic micro-organisms at the University of Minnesota-Duluth.
"We were successful in isolating an iron-oxidizing bacterium that's never been shown to be associated with corrosion before," Hicks said. "We know this organism is present in these corroded sites. We don't know just from isolating it, though, that it's the cause of the corrosion."
So there's a tantalizing correlation. But Hicks needs proof.
"So what we really need to do now is do some experimentation in the lab," said Hicks.
Even identifying the problem doesn't solve it.
"I'm confident we'll find out what's causing it," said Wisconsin Sea Grant's Gene Clark. "I just don't think we're going to find a cure-all, or a simple pill that will just stop it. I think what we'll learn is why it's happening, and we'll learn how we can lengthen the lifetime of the structures that are in place."
Researchers continue bacterial studies this summer. Others are looking at coatings that might protect steel, but can also stand up to the harbor's grinding winter ice. They have a pool of money to work with from the federal and state governments, and the university systems in Minnesota and Wisconsin.