Wassermannsloch, Karst spring cave in Eisenerz, Austria

Wassermannsloch, also called Schwarze Lacke or Neustücklgrotte, is a karst spring in the Erzbach valley east of Oberjassingau, between Hieflau and Eisenerz. Its surveyed passages total 1084 m, making it Austria's longest cave measured behind a permanent siphon. The spring lies on private land and discharges an average of 524 l/s.

The first documented exploration took place in 1747 when Joseph Anton Nagel investigated the cave on behalf of the Austrian Emperor. Since 1995, underwater passages have been explored and mapped using modern equipment.

The name refers to an old legend about a water spirit who once guided people to discover iron deposits in the area. The tale connects this place to the region's long mining heritage.

The spring lies east of Oberjassingau in the Erzbach valley, on private land, so access requires the owner's permission. The cave itself continues behind a permanent siphon and is surveyed by members of the Verein für Höhlenkunde in Obersteier within the "Aquarius" project. Discharge varies from 50 to 10,000 litres per second, so water levels change quickly.

With a surveyed length of 1084 metres, Wassermannsloch is Austria's longest cave measured behind a permanent siphon, as recorded in 2014. Its discharge averages 524 litres per second but has ranged from 50 to 10,000 litres per second. The entrance lies east of Oberjassingau in the Erzbach valley on private land.