Archaeologists are trying to piece together the mystery of an underwater trail of ancient rock piles, or cairns, that stretch for miles under the shimmering waters of Lake Constance, a glacial lake that lies between Germany, Austria, and Switzerland, and which appear to have been made by humans who lived some 5,500 years ago, according to a 2021 study.

The huge stones have attracted public attention and expert debate since they were first discovered in 2015 by the Lake Research Institute in Langenargen. About 170 such rock formations are lined up under the shallow waters of Lake Constance, a few hundred meters from its southwestern Swiss shore.

A team led by Urs Leuzinger, an archaeologist at the Museum of Archaeology of the Canton of Thurgau, have amassed compelling evidence that the rock formations were made by humans who lived in the area during the Neolithic period.

The piles are tens of meters wide and up to 2 meters high, making them impressive structures that would have taken a lot of effort and time to create, although “the function of this 10-kilometer long prehistoric feature remains enigmatic,” says a study published in the Annual Review of Swiss Archeology in 2021 Review of Swiss Archaeology).

Archaeologists have previously unearthed the remains of villages known as “pile-dwelling settlements” that were built by Neolithic peoples in the swampy area around this lake thousands of years ago. It is likely that many of these villages were involved in the construction of the stones, as the formation is too large to be the work of just one settlement.

Considering how much effort went into creating these piles, it’s interesting to consider what they might have meant to prehistoric peoples. Archaeologists are considering several possible functions, including crannogs, artificial islands built in lakes. They also suggest that the piles could have been fishing platforms, or burial sites, or border fortifications, a calendar based on the solstices, or even some sort of cult astronomical image.