Seegrotte Hinterbruhl

The Seegrotte is a gypsum mine worked from 1848 until 1912, when a blasting shot broke into a water-bearing layer and some twenty million litres flooded the lower workings overnight, creating what is described as the largest underground lake in Europe, about six thousand square metres of water in a rock chamber. Tours go through the galleries on foot and then out onto the lake in a flat-bottomed boat. The mine has a second history: in 1943 the Germans drained and used it as a dispersed factory, where forced labourers built fuselages for the Heinkel He 162, a late-war jet fighter, and a replica hangs in the chamber where the work was done. The temperature underground is around nine degrees year-round.

What you actually see

• Europe's largest underground lake - the flooded lower gallery, crossed by boat.
• A gypsum mine - worked from 1848 until the 1912 flooding stopped it.
• A wartime aircraft factory - where forced labour built He 162 fuselages.
• A replica jet fighter - hanging in the chamber where they were assembled.

When to go

All year; around nine degrees inside in any season. Two hours.

Honest expectations

Guided tours only, running to a timetable, and it is cold - bring a jacket. The wartime forced-labour history is part of the tour and is grim. Boat trips can be suspended for water levels.

Nearby

Mödling and the Vienna Woods are alongside; Vienna is half an hour north.