In the depths of Lake Constance in Europe, recent archeological work has revealed almost 200 stone structures dating back to the Stone Age. It’s not clear who built these cairns, nor why, but some recent snooping around the submerged site has looked to dig a little deeper into the mystery.

The so-called “Swiss Stonehenge” can be found in Lake Constance, a body of water that is situated where the borders of Germany, Switzerland, and Austria meet in Central Europe. 

In 2015, some 170 cairns (basically a human-made stack or pile of stones) were discovered in the southern patch of Lake Constance, arranged around 300 meters (984 feet) away from the shore in a shallow portion of that lake that has a depth of fewer than 5 meters (16 feet). Altogether, the dozens of stone piles form a 10-kilometer (6.2-mile) long underwater feature.

As detailed in a 2021 study, it’s clear these structures were not formed by natural processes and were likely crafted by humans in the Neolithic period, with some stone structures dating back over 5,500 years. 

At some point after this time, although it’s not clear when exactly, the cairns were submerged in water due to climatic change that raised the lake level by 3-5 meters (9.8-16.4 feet).

Urs Leuzinger, a researcher from the Museum of Archaeology of the Canton of Thurgau, told IFLScience that recent weeks have seen another expedition to the submerged site, which has revealed a number of new features. This included some wooden piles under the stones that are marked with cuts from a stone axe. 

Previous surveys of Lake Constance have revealed a number of submerged pile-dwellings dating from the Neolithic era that appear to have been raised above the water on stilts. Leuzinger suggests that the newly discovered cairns may have been built by the same community as lived in these pile-dwellings. It’s thought these stilted houses also date to around the year 3,500 BCE, just like the cairns. 

However, proving this link between the stones and the dwellings is extremely difficult, since researchers know next to nothing about who inhabited this area during the Neolithic era. Although we know a little about the buildings they lived in, their culture, diet, beliefs, and lifestyle remain a total mystery. 

“We still do not know why the Neolithic pile-dwellers built this 10-kilometer-long structure with around 170 cairns along the southern shore of Lake Constance,” Leuzinger told IFLScience.

He added, “Fishing traps, defense system, cleaning of the fields, cultic platform, funeral structure, astronomic calendar – who knows?!”