MADISON, Wis. - Lake Mendota continues to reveal its secrets.

And this one is the oldest of them all.

Archaeologists with the Wisconsin Historical Society announced Thursday they have identified up to nine more dugout canoes on the lake's bottom near Shorewood Hills.

The newest batch isn't as intact as the previous two canoes found in 2021 and 2022. But one fragment has been estimated through radiocarbon dating to be 4,500 years old. That would make it the oldest dugout canoe recorded in the Great Lakes region, dating back to around 2500 BC.

The canoes that were found earlier were estimated to have been 1,200 and 3,000 years old respectively and built by ancestors of the Ho-Chunk Nation. The newest cache of canoes remain on the lake bottom in pieces and aren't planned for removal, but their discovery marks another historic event for the 9,781-acre lake and is more evidence that the site, now submerged by rising lake levels, was home for millennia to an encampment by Native people.

Meanwhile, analysis conducted by the USDA Forest Products Laboratory in Madison reveals that trees used to build the canoes — elm, ash, white oak, cottonwood and red oak — changed over the centuries, signaling "environmental shifts that impacted forest composition," the Historical Society said in its announcement.

"It is an honor for our team to work alongside the Native Nations to document, research and share these incredible stories from history," said Amy Rosebrough, state archaeologist for the Historical Society. "What we thought at first was an isolated discovery in Lake Mendota has evolved into a significant archaeological site with much to tell us about the people who lived and thrived in this area."

Some of the fragments were discovered when the 3,000-year-old canoe was pulled from the lake in 2022, while others were found on subsequent dives up to last fall, said Tamara Thomsen, a maritime archaeologist with the Historical Society and who is credited with finding the first two canoes. While follow-up diving expeditions suggested the presence of additional dugout canoes, archaeologists needed time to consult with tribal partners, analyze findings and document the potentially vulnerable site before publicly releasing details, the Historical Society said.

Archaeologists hypothesize that the canoes may have been intentionally cached in the water to prevent freezing and warping in the winter months and were later buried by natural forces over time.

The Lake Mendota canoes are concentrated along roughly 800 feet of what was likely an ancient shoreline but which is now in more than 20 feet of water.

Thomsen, who owns a dive shop on University Avenue and has lived for over 20 years in a home built by her grandfather on Lake Mendota's south shore, recovered a small sample from each canoe for carbon dating, wood type analysis and other research.

"Can you imagine the history that has floated by here? It's pretty amazing," Thomsen said as she looked across the lake from her pier toward the canoe site a half mile to the east. "It's a very pleasant place to be and to dive and we enjoy doing it, so I'll keep my eyes open. You never know what the next dive is going to bring."

When Thomsen discovered the second canoe in in May 2022, a fragment from another canoe was found nearby and is being referred to as "canoe three." But when "canoe two" was floated to the surface, another canoe was found underneath it and is being called "canoe four."

Any wood nearby that appeared to be worked by hand was brought to the surface and joined canoe two in a trip to the State Archive Preservation Facility on Madison's Near East Side. That's where they were photographed, scanned and then placed in the same vat of purified water and UV sterilization as the 1,200-year-old canoe found in 2021.

It was unclear at the time if the fragments were from other canoes or were parts of canoe two or three, but testing revealed the pieces came from different canoes, Thomsen said.

"It's crazy. You just keep looking and there's more and more and more," Thomsen said. "And as we test them, they have a unique enough profile that they are different."

There's never been a systematic search via diving, but in late 2022 and early 2023, tribal historic preservation officers Bill Quackenbush of the Ho-Chunk Nation and Larry Plucinski of the Bad River Band of Lake Superior Chippewa joined members of the Historical Society to walk the frozen lake with ground-penetrating radar looking for more potential evidence in the area where the first two canoes were found.

The research continued last week when Quackenbush, who focuses on heritage preservation and specializes in GPR technology to research ancestral sites, lashed his own dugout canoe equipped with GPR to Thomsen's boat, which then pulled the canoe back and forth across the lake's surface above the site where the canoes had been found.

The analysis of that work has not yet been completed, but archaeologists have determined that the additional canoes are not physically intact enough to withstand recovery by divers and the process necessary to preserve the canoes.

"We are excited to learn all we can from this site using the technology and tools available to us, and to continue to share the enduring stories and ingenuity of our ancestors," Quackenbush said.

The four oldest of the canoes date back to the Late Archaic period between 2,500 and 5,000 years ago when hunters and gathers lived in seasonal communities. Three are made of elm and were estimated to have been built between 3,700 and 4,500 years ago while the other was the second canoe found in 2022. It was built from an ash tree about 3,000 years ago.

Two of the canoes — one made from cottonwood, the other from white oak — date to the Middle Woodland period from between 1,500 and 2,000 years ago when trade networks were being expanded and early long-term communities were being formed. Up to four canoes date to the Late Woodland period from about 900 to 1,500 years ago and were made of white oak. The most recently built canoe is from the Oneota period from about 800 years ago when permanent farming towns and smaller communities were the norm. It is the only canoe in the cache made of red oak.

While research continues at the Lake Mendota canoe site, the two recovered canoes remain in the Historical Society's archives facility where they are undergoing a preservation process that uses polyethylene glycol to stabilize the wood. The treatment began in February 2024 and is expected to conclude in 2026. Once the PEG application is complete, the canoes will be transported to Texas A&M University to undergo a freeze-drying process that leaves the canoes in a stable, solid structure suitable for public display.

That's expected to be in 2027, when the Historical Society's new Wisconsin History Center opens on Capitol Square. But for now, the largest lake in the Yahara River chain will continue to be studied.

"We have a lot to learn from the Mendota canoe site," Plucinski said. "The research happening today allows us to better understand and share the stories of the people who lived here and had a thriving culture here since time immemorial."