The discovery of a huge system of underground corridors raises many questions and even makes previous historiography shake. During renovation work, a centuries-old cannonball was found in the roof truss of a farmhouse. Inside it, a plan is discovered that points to a labyrinth of underground passages - this is not the beginning of a thriller, but really happened in the East Styrian community of Vorau. And it was the starting point of a series of almost unbelievable discoveries. The speleologists Ingrid and Heinrich Kusch set out to explore the many kilometres long and apparently deep corridor system and in doing so repeatedly came across details for which there is no explanation. Most of these passages are not only bricked up after a certain distance, but also meter-thick with tons of water.