A stone wall almost a kilometer long has been discovered 21 meters below the surface of the Baltic Sea, off the coast of Germany. The wall is thought to have been built around 11,000 years ago to channel reindeer to places where they could be more easily killed, and may constitute Europe's largest Stone Age megastructure.
The discovery was made by chance. In 2021, students in training exercise with geophysicist Jacob Geersen at the Leibniz Institute for Baltic Sea Research, Warnemünde, Germany, used multibeam sonar to map the seafloor 10 kilometers offshore from the town of Rerik.
“In the lab, we realized that there was a structure that didn't look natural,” says Geersen.
So in 2022, he and his colleagues lowered a camera to the structure, which revealed a row of stones. “It was only when we contacted archaeologists that we realized it could be something important,” says Geersen.
There is no reason or evidence that a modern structure was built underwater at this site, a team member says. Marcel Bradtmoller, archaeologist at the University of Rostock, Germany. The team also can't think of a natural process that could create such a structure.
This suggests that the wall was built when this area was dry land, meaning it must be between 8,500 and 14,000 years old, says Bradtmöller. Before this, the area was covered by an ice sheet that would have destroyed any stone structures, while, later, rising sea levels submerged the area.
The wall runs along what was once a lake. It contains around 10 large rocks measuring up to 3 meters in diameter and weighing several tonnes, connected by more than 1,600 smaller stones mostly weighing less than 100 kilograms. The stones are placed next to each other rather than on top of each other, and the wall is less than a meter high in most places.
The large stones are all found where the wall zigzags or zags. The team therefore believes that the structure was built by connecting large stones that were too heavy to move with smaller stones that could be moved.
Bradtmöller thinks it was probably made by hunter-gatherers belonging to what is known as the Kongemose culture, named after a site in Denmark where objects such as stone tools were discovered.
The most likely explanation is that the structure was used to channel reindeer, he said. “The hypothesis that, at the moment, is most suitable is that of a training wall for hunting.”
Although these hunter-gatherers are thought to have lived and moved in small groups, they may have gathered in larger numbers at the lake when reindeer arrived in the area, says Bradtmöller.
Similar low walls, sometimes called desert kites, have been found in many places in Africa and the Middle East, as well as under the Great Lakes in North America. Some are up to 5 kilometers long and it is now widely believed that they were used for hunting.
Although these walls are usually low enough for animals such as antelopes to jump over, they usually avoid them when running in herds, explains Marlize Lombard at the University of Johannesburg in South Africa, which discovered similar structures. “In such circumstances, they tend to run parallel to obstacles such as low fences, instead of crossing them,” she explains.
Many desert kites consist of two V-shaped walls to channel animals, but a single wall can still provide an effective guideline, Lombard says. One possibility with the newly discovered wall is that it was used to drive reindeer into the lake, where they were hunted from boats, says Bradtmöller.
It's also possible that there is a second sediment-covered wall nearby, Geersen says. He plans further investigations, including diving, to try to find direct evidence of Stone Age individuals, but so far researchers have been thwarted by bad weather.
Other experts also agree with their findings. “I think the arguments for the wall are very valid as an artificial structure built to channel the movements of migratory reindeer,” says the archaeologist. Geoff Bailey at the University of York in the United Kingdom.
“Such a discovery suggests that vast prehistoric hunting landscapes could survive in a way previously reserved for the Great Lakes,” explains Vincent Gaffney at the University of Bradford in the United Kingdom. “This has very big implications for areas of coastal shelves that were previously habitable.”
Modern activities such as trawling, cable laying and wind farm construction can destroy such sites, says Geersen. More in-depth explorations are therefore necessary to find them before they disappear.
No other structures of this type have been discovered in Europe, says Bradtmöller. He thinks it is likely that many of them once existed, but were destroyed by human activities.