Dlouhá stoka (Long Ditch) is a water ditch intersecting a large part of Slavkovský les (Slavkov Forest). It flows from Kladský rybník (Kladská Pond) at an elevation of 820 metres above sea level, it is 24.2 km long, and the bed is of a trapezoid shape with a width ranging from 1.2 to 3 metres. It is laid with stones. The water canal roughly heads from the west to the east. The purpose of building the ditch was to supply water to tin mines in the vicinity of the towns of Horní Slavkov and Čistá where one of the richest tin deposits in Europe were located at that time.
The building of the Long Ditch is credited to Jan Pluh of Rabštejna. Construction commenced in 1531 and the waterworks were completed in 1536. The route was staked out by mine surveyor Rossmeisel and the waterworks were named Flossgraben.
As mining was consuming more and more water for driving mining machinery and equipment, and the supplies of wood in the surroundings of Horní Slavkov and Krásno were thinning, it was necessary to transport wood to the mining areas from the distant Kynžvart forests. The Long Ditch served, among other things, for this particular purpose.
In the second half of the 16th century, the water system was continuously improved and it constituted a network of canals more than 30 km long. It also included a system of 10 large mining ponds. The entire water system supplied water to 52 ore mills, passing under 35 bridges and through 14 sluice-gates. Its average decline was approximately 0.35 m per 100 metres and the ditch was more than 2 metres wide. The waterworks were unprecedented in terms of size or design in 16th century Central Europe. Similar canals were built in Harz, Germany, and in the district of Banská Štiavnica in Slovakia much later.
The Long Ditch was partially built on older artificial waterworks. It seems that people had attempted to exploit water in present-day Kladská long before Rössmeissel. An agreement on the purchase of a meadow by the small river of Roda dating to 1408 suggests that the first ditch had been built in the area in the late 14th century. It leads from the Roda along a level line. Whether it served only miners in Žamberk (as Sangerberg was called then) or whether it continued further, remains unknown. Nevertheless, an agreement concluded on the 10th of May 1499 between mine-owners and the nobility on the use of a water ditch clearly implies that a water connection up to Krásno had already existed.
The Long Ditch was repaired and maintained even after the decline of mining activities after the Thirty Years War. In 1872, the Flossgraben (the German name of the waterworks) Society was established and it administered, cleaned, and managed the waterworks until World War II.
At present, the Long Ditch is a popular destination of hikers. The unique waterworks were declared a cultural monument in 2003.