Is It an Octopus? a 4-Year-Old Asks. No, a Shipwreck From 1871.

It began as a typical summer outing for Tim Wollak and his daughter, Henley, 4. On a clear morning on Aug. 13, they set out in their 22-foot boat from the eastern shore of Wisconsin. The sky was blue and the waters flat and calm, perfect conditions for them to explore the shallows of the bay and look for walleyes, large-eyed game fish common in Lake Michigan.

But then, about three hours into their jaunt, Mr. Wollak, a 36-year-old medical devices salesman, and Henley, who was two days away from her 5th birthday, found their lives intersecting with history.

A shipwreck from the 1871 Peshtigo fire was about to reveal itself.

As they chugged along the shoals of Green Island, their boat’s sonar delivered images of shadows, sand and indistinct rock features on the bay floor about 10 feet below them. Then a cluster of long, slender objects edged into view, forming a pattern far too regular to have been fashioned by erosion or waves. Mr. Wollak turned to his crew of one.

“I immediately said, ‘Henley, come over here and take a look at this. What do you think it is?’” he said in a recent interview. “She thought it was an octopus.”

The Great Lakes served as a vital commercial hub in the 19th century, providing a shipping passageway to the East Coast via the Erie Canal. More than 3,000 vessels have been lost in Lake Michigan, said Brendon Baillod, the president of the Wisconsin Underwater Archaeology Association.

Mr. Wollak said he had occasionally come across lost vessels, mostly an odd row boat, but he had never before seen one so large. He navigated around the spot, trying to capture a better view of the ship, which was partially buried. He took a photograph of the sonar image and posted it online, marveling at the frame’s radiating ribs that his daughter had interpreted as tentacles.

On Dec. 11, historians in Wisconsin looked at the images and identified them as most likely the wreck of the George L. Newman, a 122-foot-long wooden vessel with three masts that sank in 1871.

“The ship was abandoned, became covered with sand, and was largely forgotten — until it became exposed and was located by the Wollaks this past summer,” the Wisconsin Historical Society said in a statement.

The Wollak family lives in Peshtigo, in northeastern Wisconsin, an area that thrived in the 19th century thanks to the logging industry, but also faced fires and smoke from the industry’s practices.

On Oct. 8, 1871, the George L. Newman, carrying lumber, was enveloped in smoke billowing from an onshore fire worsened by dry conditions. The Peshtigo fire eventually burned through 1.2 million acres in the Northeastern and Upper Peninsula parts of Michigan, and about 2,000 people were estimated to have died, making it one of the deadliest fires in U.S. history.

The smoke was so dense that a lighthouse keeper kept the light on during the day on Green Island in the bay, but the George L. Newman became grounded on the island’s southeast point, the Wisconsin Historical Society said. The crew was rescued, but the ship was lost.

After the historical society’s Maritime Preservation and Archaeology Program spotted Mr. Wollak’s photographs, it teamed up with conservationists from the Department of Natural Resources to investigate the wreck using a remotely operated vehicle, and concluded that its location matched what was known about the ship’s fate.

Another survey to confirm it for listing on the National Register of Historic Places is planned in the spring of 2024, the historical society said.

But Mr. Baillod said he was confident that the George L. Newman had been found. The accidental discovery of wrecked ships, or whatever was left of them, in the lake’s clear waters has become easier with Google Earth and sonar, he said.

“People often run over something and think it’s an old piece of building or part of a dock and actually it’s a historical ship,” he said.

There are no octopuses in the Great Lakes, however.

“Never have been,” he said.