The Petrus Tunnel is a hereditary tunnel, a tunnel built beneath the actual ore extraction tunnels for drainage purposes. It was intended to divert water from a series of older mine workings in the Lochner Mountains into the Zwodau River. The tunnel is 711 fathoms long, or at least that's how it is recorded in historical sources. Based on the then-standard Viennese fathoms of 1.8965 meters, the tunnel is 1.35 kilometers long. The Petrus Tunnel allowed ore mining to continue in an area almost as far as Bleistadt (Olovi), 2.2 kilometers away.