Submerged wall could be the largest Stone Age megastructure in Europe

A low stone wall nearly a kilometre long has been found 21 metres below the surface of the Baltic Sea off the German coast. The wall is thought to have been built around 11,000 years ago to channel reindeer into places where they could more easily be killed, and could be the largest Stone Age megastructure in Europe.

The discovery was made by chance. In 2021, students on a training exercise with geophysicist Jacob Geersen at the Leibniz Institute for Baltic Sea Research Warnemünde in Germany used a multibeam sonar to map the seafloor 10 kilometres offshore from the town of Rerik.

“Afterwards, in the lab, we realised that there was this structure that looks not natural,” says Geersen.

So in 2022, he and his colleagues lowered a camera down to the structure, which revealed a row of stones. “It was only when we contacted the archaeologists that we understood it could be something significant,” says Geersen.

There’s no reason or evidence for a modern structure to have been built underwater at this site, says team member Marcel Bradtmöller, an archaeologist at the University of Rostock, Germany. Nor can the team think of any natural process that could create such a structure.

This suggests the wall was built when this area was dry land, meaning it must be between 8500 and 14,000 years old, says Bradtmöller. Before that, the area was covered by an ice sheet that would have destroyed any stone structure, while, later, rising sea levels submerged the area.

The wall runs alongside what was once a lake. It contains around 10 large rocks up to 3 metres across and weighing several tonnes, connected by more than 1600 smaller stones mostly under 100 kilograms in weight. The stones are placed next to one another rather than on top of each other, and the wall is less than a metre high in most places.

The big stones are all found where the wall zigs or zags. So the team thinks the structure was built by linking large stones that were too heavy to move with smaller stones that could be shifted.

Bradtmöller believes it was probably made by hunter-gatherers belonging to what is known as the Kongemose culture, named after a site in Denmark where artefacts such as stone tools have been found.

The most likely explanation is that the structure was used to channel reindeer, he says. “The hypothesis that, at the moment, fits best is a driving wall for hunting.”

While these hunter-gatherers are thought to have lived and travelled around in small groups, they might have assembled in larger numbers at the lake when reindeer came to the area, says Bradtmöller.

Similar low walls, sometimes called desert kites, have been found in many places in Africa and the Middle East, and also beneath the Great Lakes in North America. Some are up to 5 kilometres long, and it is now widely agreed they were used for hunting.

Although these walls are typically low enough that animals such as antelope could jump over them, they usually avoid them when running in herds, says Marlize Lombard at the University of Johannesburg in South Africa, who has discovered similar structures. “In such circumstances, they tend to run parallel to obstacles such as low fences, instead of traversing them,” she says.

Many desert kites consist of two walls in a V-shape to funnel animals, but a single wall can still be an effective driving line, says Lombard. 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 is also possible that there is a second wall covered by sediment nearby, says Geersen. He plans further investigations, including diving, to try to find direct evidence of Stone Age people, but, so far, the researchers have been thwarted by bad weather.

Other experts also agree with their conclusions. “I think the case is well made for the wall as an artificial structure built to channel movements of migratory reindeer,” says archaeologist Geoff Bailey at the University of York in the UK.

“Such a find suggests that extensive prehistoric hunting landscapes may survive in a manner previously only seen in the Great Lakes,” says Vincent Gaffney at the University of Bradford in the UK. “This has very great implications for areas of the coastal shelves which were previously habitable.”

Modern activities such as trawling, cable-laying and wind farm construction can destroy such sites, says Geersen, so more exploration is needed to find them before they are lost.

No other structures of this kind have been discovered in Europe, says Bradtmöller. He thinks it is likely that many once existed, but they were destroyed by human activities.