Hardtstollen, Mining tunnel entrance at Hardtberg, Wuppertal, Germany

The Hardtstollen is an adit driven about 250 metres into the Hardtberg in Wuppertal, from an entrance on the Hardtufer street at the border of Barmen and Elberfeld. It runs beneath the Bismarck Tower, where the rock cover is 60 to 80 metres thick. At its end lies the Hardtkaverne, a chamber about 20 metres high, partly supported by steel beams and corrugated sheets.

Construction of the tunnel started in the 1960s as part of a planned energy storage project designed to manage peak power demand in Wuppertal. The project was later abandoned, yet left behind this underground structure.

The Arbeitskreis Kluterthöhle has officially looked after the adit since 2015 and opened it to visitors during the 2015 Tag des Geotops, when almost 200 guests joined a special tour. The same event was repeated in 2016, 2025 and 2026. The cave cut by the tunnel, the Tiefe Hardthöhle, was surveyed after the agreement and its known length grew from 5 to over 100 metres.

The site is maintained by a local group and open for guided tours on specific days, mainly during an annual geological event in September. It is wise to check beforehand about access and opening times.

The Hardtkaverne was meant to hold aircraft turbines, and the exhaust was to be routed up to the Bismarck Tower, which would have served as its chimney. The chamber has a clear height of about 20 metres. The project was abandoned because the rock did not yield enough water to run the plant.