Kermeterstollen, Underground water tunnel at Heimbach power station, Germany

Kermeterstollen is an underground water tunnel running about 2.7 kilometers through the Kermeter ridge in North Rhine-Westphalia. It carries water from the Urft reservoir to the Heimbach hydroelectric power station.

Built in 1905 together with the Urft dam and the Heimbach power station, the tunnel was driven from both ends and the two headings met with less than one metre of deviation. On 10 February 1945 German troops blew up only the tunnel's seals at the Heimbach power station; the reservoir then drained as far as the tunnel level.

The tunnel's name links it to the Kermeter ridge it passes through; it is also called Urftstollen after the reservoir it drains. Its two valve houses are the visible markers of the system: the one above the Südschacht holds a 7,000 kg shut-off slide valve installed in 1905 and replaced in August 2002, while the outlet house above Heimbach contains two separate valves for the penstocks.

The tunnel is not normally open to visitors as it is part of an active power station facility. Professional inspections occur every 20 years and require the Urft reservoir to be drained for full access to the tunnel interior.

Construction teams met underground with less than one meter of deviation despite having 220 meters of rock above them. This precision was achieved using surveying techniques from the early 1900s without modern electronic equipment.