Below the surface of Lake Superior rested the legend of a diving bell.

For decades, it was yet another mystery surrounding the storied Congdon family of Duluth — a seemingly tall tale that most dismissed as fiction.

But when a massive concrete and steel diving chamber weighing about 1 ½ tons was discovered this week on the grounds of the Glensheen mansion, it only added to the intrigue surrounding the infamous mansion and those who lived there.

For now the staff at Glensheen, a sprawling London Road historic estate that has since been turned into a museum, are reveling in the unlikely find.

“This is one of the coolest things we have historically uncovered because it speaks to the port history in Duluth,” Dan Hartman, Glensheen’s director, said Friday. “When people think of Minnesota they don’t think of a diving bell being used.”

What’s more, Hartman said, it’s hard to imagine why the wealthy Congdon family would own a bell that could lower a human into the depths of the lake.

“It makes sense if a university would own one or if the [Duluth Seaway] Port Authority had one,” he said. “But why would a private family own a diving bell?”

Hartman and his staff are trying to find those answers as well as learn more about diving bells. They’ve contacted marine experts and publicized their find in hopes of jostling the memories of Congdon relatives who are scattered across the country.

The discovery, inside the mansion’s boathouse, caught museum staff by surprise as crews worked to restore the only remaining structural boathouse on Lake Superior.

The large concrete building, which once housed Chester Congdon’s 54-foot yacht, was battered by a 2017 storm that filled the boathouse channel with a massive amount of rock.

Until the storm hit, the channel was filled with water. Three metal poles poked through the surface, and on a nice day, the Glensheen staff could see rivets that appeared to be on a metal drum.

As heavy equipment removed rock during the recent restoration work, crews pulled up the metal drum and finally separated fact from fiction. It wasn’t until it was moved from the shadows of the boathouse, however, that the museum staff’s skepticism about a diving bell was erased. Now, they’re pushing to piece together the story behind it.

Hartman said they long heard tales that it belonged to Clara and Chester Congdon’s nephew, Alfred Bannister. He came to live with the family when his parents died in the 1900s, Hartman said. He eventually went to Cornell College and Massachusetts Institute of Technology (MIT) and became an engineer who loved science and exploring, Hartman said.

When Chester Congdon needed his yacht transported from New York, where it was built, to Duluth, Bannister volunteered to sail it across the Great Lakes, he said.

“It was such a feat at the time that his journey was covered by a national yachting magazine,” Hartman said. “That’s why this diving bell story does sound like Alfred Bannister.”

The bell, an enclosed steel chamber with a concrete bottom, is about 8 feet high and 5 feet wide, Hartman said. One person could fit comfortably in it while air is pumped inside. The bell was probably used between 1920 and 1940, Hartman said.

“The story goes that [Bannister] used it to check on the pier’s foundation,” he said. At least, that’s according to the tale that’s been told.

“Family legends don’t often pass down well,” Hartman said.

The oddity of the Congdon family owning a diving bell is intriguing to Hartman and others. History is filled with stories of the peculiar and sensational, Hartman said.

“And this is pretty sensational,” he said. “And the fact that it was hiding in this boat channel for how many umpteen decades is exciting.”

But so is the fact that there are so many questions yet to be answered, Hartman said.

“That’s a whole new mystery to try to solve. … It’s also this cool thing to see. There’s a slew of people who are excited about this piece,” Hartman said.

In fact, one person offered Friday to buy the diving bell for $10,000, Hartman said.

It’s not for sale. Instead, Hartman’s staff wants to find the right home where it can be preserved and serve as an educational piece.

“I want it to [stay] in the Duluth area, connected to the port story,” Hartman said. “You just don’t see these things in Minnesota.”