Lisa Sonnenburg cradles a stone flake about the size of a fingernail in the palm of her hand.

"This is one of our best ones," said the geoarcheaologist at the University of Michigan. "What's cool about this is you can actually see where it's been split off from the rock."

To Dr. Sonnenburg and her colleagues, the fragment bears the mark of a human maker. What makes that interesting – and controversial – is that the flake was scooped up last summer from a layer of 9,000-year-old sediment at the bottom of Lake Huron. The group contends it is one of many signs that point to the existence of a submerged hunting complex that is more than twice as a old as Stonehenge and the Great Pyramids of Egypt.

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This week, the team was back out on the water looking to bolster a theory that ancient caribou hunters once occupied a bridge of land that spanned the middle of the lake. If they succeed, they'll have established a direct link to one of the most shadowy periods in prehistory.

"We don't know a lot about these people," Dr. Sonnenburg said. "That's what makes it exciting."

Archaeological evidence from various locations suggests that people have been living in North America for at least 13,000 years, when an ice age population known as the Clovis culture was widespread across the continent. At some point, that population transitioned into diverse groups of hunter-gatherers that are the ancestors of present-day native Americans, but little is known about this transition period.

It was during this time that Great Lakes were emerging from under the receding glaciers, and rebounding bedrock caused Lake Huron's water level to drop 100 metres below where it sits today. This exposed the 200-kilometre-long Alpena-Amberley Ridge, which stretches eastward from the Michigan side of the lake to Point Clark, near Kincardine, Ont.

The existence of the ridge as an uninterrupted structure was not fully appreciated until 2007, when it appeared in an updated U.S. government map. John O'Shea, a professor of anthropology at the University of Michigan, noticed the feature and reasoned that it would have made a convenient corridor for herds of caribou, which are known to have the roamed post-glacial landscape around the lake. And where there were caribou, hunters may have been waiting to meet them.

"The animals have pretty predictable movements," Dr. O'Shea said. "And the narrowness of the ridge adds to that predictability."

Beginning in 2008, Dr. O'Shea assembled a group to search the ridge for signs of human activity using side-scanning sonar. Dr. Sonnenburg, a Canadian researcher, joined soon after.

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Then, in August, 2013, came Drop 45 – named for the 45th time the sonar was deployed, at a spot about 50 kilometres from the Michigan shore.

"We hit on this one and, bang, we all knew that we had found something," Dr. Sonnenburg said.

That something was a series of rock structures that resemble the guides and blinds indigenous people once used in the Canadian North for directing herds of caribou so that hunters could efficiently kill a large number of animals in a short time. It is around these structures, and nowhere else, that the stone flakes have turned up. Radiocarbon dating of preserved wood and charcoal found nearby suggests they are the oldest hunting structures found anywhere in the world.

On dry land, generations of activity, both native and European, would likely have removed or disturbed these clues. The team's findings suggest they have survived untouched for millenia in the lake's frigid depths.

To Dr. O'Shea, the evidence suggests a people that had to organize themselves into groups large enough to manage the seasonal flood of caribou meat.

"What we think we're seeing is a pattern where small groups of hunters are dispersing in the winter, and in the spring they're aggregating at these key locations and gradually building up these structures," Dr. O'Shea said.

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The Drop 45 findings, published in April, quickly attracted media attention, but some archaeologists remain skeptical of the Michigan team's interpretation of what it has found. Scarlett Janusas, an independent archaeologist based in Tobermory, Ont., praised the group's efforts but added that she is not yet "100 per cent convinced" by the evidence.

"What is needed is definitive proof of cultural activity," she added. Another colleague gave Dr. O'Shea a bottle of Scotch with instructions that it not be opened until he had found indisputable evidence of people living on the ridge.

Now, using an underwater excavation technique known as "airlifting," the team has started systematically vacuuming small sections of the lake bottom to gather what may have been deposited there. During one day last weekend, they pulled up as much sediment as during all previous trips combined. The sediment is screened and carefully examined for evidence of what or who may have once lived on the ridge.

So far, the team says what it has found this year fits with the caribou hunting scenario, but the real hunt is just beginning and could eventually lead across the lake.

"It's a huge area and there's so much more," Dr. Sonnenburg said. "Ideally, I'd like to get back and look at the Canadian side and fill in the gap on that end."

In the process, an early North American society may finally emerge from the waters – a telling reminder that humanity's presence here is as old and expansive as the lakes themselves.