Archaeologists in Germany have uncovered a late medieval underground tunnel hidden within a much older Neolithic burial site near Reinstedt.

The discovery was announced Jan. 29 by Saxony-Anhalt’s State Office for Heritage Management and Archaeology after excavations ahead of wind turbine construction.

The burial complex dates to the Baalberge culture of the 4th millennium B.C., while the tunnel, known as an erdstall, was built about 5,000 years later in the Middle Ages.

Finds such as medieval pottery, a horseshoe, charcoal, and animal remains suggest the passage was briefly used and then intentionally sealed, though its purpose remains unclear, Fox News has reported.

Researchers speculate the tunnel’s placement may reflect medieval avoidance of pagan burial grounds and adds to a wave of recent archaeological discoveries across Germany.