Henley Wollak has a lively imagination.

Every night at bedtime, the 5-year-old becomes a sea creature, tucked beneath a bedspread decorated with a massive mermaid tail, her human legs hidden under the covers.

A fairy tale came true for Henley last summer on the waters of Lake Michigan when the Peshtigo, Wisconsin, girl discovered a 150-year-old shipwreck.

Henley and her dad were boating toward her favorite island beach when they saw something under the surface that Henley thought was an octopus. Her dad, Tim Wollak, knew that hundreds of vessels had been lost to the lake over the years that were never recovered. He thought the long grey lines below the water looked like the hull of a ship. He later posted photos of their mysterious underwater discovery on Facebook.

“At first when we saw it in the boat, I thought it was cool to see something like that," Wollak, 36, told USA TODAY. "At that time I didn’t know the magnitude of what it was.”

This month the Wisconsin Historical Society delivered news that was beyond the father and daughter's wildest dreams: They had discovered a long-lost 150-year-old shipwreck.

Researchers said last week they believe the ship Henley and her dad spotted is the remains of the George L. Newman, a vessel built before the Civil War. Historical records show that it sank in 1871 after struggling to navigate through heavy smoke from the Peshtigo Fire, the deadliest forest fire in U.S. history.

Until this year, the Newman had never been spotted, researchers told Wollak. As the Wisconsin Historical Society explained in a Facebook post, 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."

“I was blown away by it, especially being able to share it with Henley," Wollak said.

Father-daughter outing led to discovery

On Aug. 13, Henley asked her dad if they could go swimming off the southeast shore of Lake Michigan's Green Island. She wanted to look for rocks and sea glass. Her dad wanted to go fishing, he told USA TODAY.

The two struck a compromise. Wollak agreed to take Henley to her favorite spot, driving their motorboat boat "real slow" along the route there, to look for fish.

That's when they saw what they later learned was the George L. Newman. Wollak gave his daughter full credit for placing them where they were when they made the find.

“She ultimately put us in that location because that’s where she wanted to swim," Wollak said.

Maritime archaeologist Tamara Thomsen told FOX 11 News in Green Bay that local researchers "didn't have anything" about a shipwreck documented in the area where the Wollaks found the George L. Newman.

"It's a pretty significant shipwreck, pretty old for Wisconsin shipwrecks," Thomsen said about the vessel. Historical records say the vessel was more than 40 yards long and 9 yards wide.

Wisconsin shipwreck sank in 1871

Officials from the Wisconsin Department of Natural Resources later went out to Henley's spot on the lake to capture sonar scans of the wreck. Historians said the ship on the lake bottom matched the measurements and best-estimated location of the wrecked George L. Newman.

The ship was built in 1855 in Ohio, and it was a three-masted barkentine, akin to a schooner, according to the Wisconsin Historical Society. On its last voyage in October 1871, the Newman was transporting lumber from Little Suamico, Wisconsin, according to the society.

Historians said the ship was sailing blind that day through thick smoke from the Peshtigo Fire, a forest fire that surrounded Lake Michigan's Green Bay and killed more than 1,200 people, according to the National Weather Service.

"To have it tied to the Peshtigo fire makes it even more special," Thomsen, the archeologist, told FOX 11 News.

The smoke was so dense that the Green Island lighthouse keeper left the light on throughout the daylight hours, which is when the George L. Newman ran aground on the southeast point of Green Island, the historical society said. The lighthouse keeper rescued the crew, who stayed at the lighthouse for a week, and they salvaged what they could from the wrecked vessel.

Henley, who recently turned 5, may not completely understand the historical significance of her discovery, her father said.

“She has such a love for the outdoors, I think as she gets older the magnitude of this will sink in for her," Wollak said.

For now, Henley is most excited for next summer, when lake waters will be warm enough to go swimming again. Fishing is growing on her too.

And gazing out of her dad's boat down into the sparkling waters of Lake Michigan will always hold a special place in her heart.