Main Portal of the Rothschönberger Stolln

"The Rothschönberger adit is a water solution adit in the Brander and Freiberg mining area.
It was driven from 1844 to 1882 and, with its ancillary facilities, has a total length of 50.9 km and is equipped with 8 light holes. The average gradient is 0.033%. It is navigable by boats for inspection and maintenance. The mouth hole, which allows the water to flow into the Triebisch, is in the municipality of Klipphausen near Rothschönberg. The water flow is 685 liters per second. The construction costs amounted to 7,186,697.43 Reichsmarks and exceeded the estimate by 79%.
Of the original eight light holes, three are still preserved today with their hat houses. In addition to the fourth light hole near Reinsberg, these are the seventh and eighth light holes near Halsbrücke. From the other light holes the shaft buildings, from the II. and III. Lichtloch also removed the heaps.
The Rothschönberger Stolln is part of the UNESCO World Heritage Montanregion Erzgebirge." - de.wikipedia.org/wiki/Rothsch%C3%B6nberger_Stolln

Very hidden but worth a visit.