In the fall of 2021, Jacob Geersen, a marine geologist now at the Leibniz Institute for Baltic Sea Research, was teaching a one-week field course at the University of Kiel. The class was conducted entirely aboard a research vessel on the Baltic Sea.

Geersen prefers the open-air classroom. "It's quite intense," he says, but for some of the students, "it's maybe the best time during their studies."

During the night shift each evening, students mapped the shape of the seafloor at high resolution. "Usually if we go somewhere and do these measurements," says Geersen, "then we find something interesting." This research cruise proved no exception.

One night, in the Bay of Mecklenburg, off the coast of northern Germany, the students fired up the echosounders and mapped a swath of seafloor. "The next day, we downloaded the data," says Geersen. "And it was then when we were sitting together, we saw that there was something on the seafloor. It was something special."

They didn't know it at the time, but not quite 70 feet below the surface, they'd stumbled upon a stone wall more than half a mile long that dated back to the Stone Age — one of the oldest such megastructures on the planet. In research published in PNAS, Geersen and his colleagues say this piece of ancient hunting architecture may have been used to corral and hunt reindeer, adding a level of sophistication to the prehistoric hunter-gatherers who lived 10,000 to 11,000 years ago.

The Blinkerwall is revealed

Geersen was used to seeing rocks and stones show up on the echosounder as bumpy anomalies scattered across the bottom of the Baltic Sea, left behind when the glaciers retreated from northern Europe thousands of years ago. But back aboard that vessel in the Bay of Mecklenburg, he could already tell that what he was seeing was different.

"You saw there is something that kind of meanders through the map," says Geersen. It was a ridge that ran for six tenths of a mile. "I thought it's very likely that these are rocks, one next to the other, lined up," he says.

A year later, Geersen, his colleagues and a new batch of students returned to that same site. They lowered a camera down, and confirmed this ridge was made up of thousands of rocks that formed a kind of wall standing about 1.5 feet tall on average.

"It's usually small stones — like tennis or soccer ball size — so movable stones," says Geersen. "But then at some places where we have a large stone, the direction of the wall changes."

Geersen didn't know how such a structure, which the researchers dubbed the "Blinkerwall" after a nearby underwater mound called Blinker Hill, could have formed naturally.

"It was only when we went to the archaeologists that they said, 'You may have found something very significant,'" he says.

"I was probably the most skeptical of the entire team," recalls Berit Eriksen, a prehistoric archaeologist at the University of Kiel who studies the people who arrived in northern Europe when the glaciers retreated after the last Ice Age 10,000 to 20,000 years ago. When she examined the structure from the Bay of Mecklenburg, a line from Sherlock Holmes came to mind: "When you have eliminated the impossible, whatever remains, however improbable, must be the truth."

"Archaeologists never speak of 'truth,'" admits Eriksen, "but I'm running out of things to eliminate in terms of natural stuff. That's my problem."

Eriksen reviewed the data and became increasingly convinced that the structure was made by prehistoric humans who'd used lots of smaller stones to connect the larger unmovable rocks into a wall. "I don't believe in UFOs, so it's got to be manmade," she concludes. She and the other archaeologists on the project agreed that the wall was likely used by hunter gatherers 10,000 to 11,000 years ago during the Stone Age to help them herd and hunt reindeer by the hundreds.

How to hunt hundreds of reindeer in the Stone Age

"The only way you can kill this amount of reindeer is if you drive them into a shooting blind, if you cut them off at the pass somewhere," explains Eriksen. And reindeer are known to follow these kinds of stone walls naturally, even stout ones like the Blinkerwall.

"There would have been water at the other side," says Eriksen. So the reindeer would have become trapped between the wall and the water, allowing the hunters lying in wait to fire their arrows at the reindeer. Eriksen says these prehistoric people were nomadic, but this wall suggests they may have had a regular migration route, one that would have brought them back to this spot year after year.

"If you build a structure like that," says Eriksen, "you're someone who knows the entire area extremely well. You're not just moving around an unknown landscape. You don't just hope you can find a reindeer that day. You plan. You know where the reindeer will come next year." It's a theory that archaeologists have kicked around for a while, but she says this wall helps confirm it may have been true in prehistoric Europe.

Ultimately, the area was flooded, forming the Baltic Sea we know today and submerging this piece of hunting architecture under the water.

Ashley Lemke, an underwater archaeologist at the University of Wisconsin, Milwaukee, was not involved in the study. She said the research was strong — and performed under challenging circumstances.

"I know this personally — working underwater is not easy," says Lemke, who has discovered similar stone walls in Lake Huron, one of the Great Lakes beside Michigan.

Lemke explains that these results reinforce the argument that people living during the Stone Age were more sophisticated and nuanced than we tend to give them credit for. "We always think of them on the brink of starvation, trying to scrape a living out of the landscape. And that's just not true," she says. Instead, "people in Europe were building things before Stonehenge, before these more classical structures that we think of."

"This is actually really early examples of almost animal domestication," Lemke continues. "Like before you start keeping animals in pens permanently, you're kind of making fences to hunt them, which I think is really interesting." This practice may have eventually led to livestock herding.

To confirm this wall was made by prehistoric people and used to hunt, the researchers will need more archaeological evidence of hunting-related activity. Berit Eriksen says such clues should be there, given the hunters would have had to wait for the reindeer to show up.

"You'd have to eat while you're there, so you can see if there are small bits of charcoal," she says. It may be possible to excavate arrowheads or ancient DNA. In addition, "they would have defecated," Eriksen says. "So you can find stuff — traces of people — if you're lucky."

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