Here’s the largest underground lake in Europe, the Seegrotte. It is situated in the village of Hinterbrühl not far from Vienna. The lake is an important historical site, used to be a mine where combat aircraft such as the world’s first jet fighter, the Heinkel He162, were constructed.
Stamps used include the Steinadler (Golden eagle), the Wörthersee Kärnten lake, and the EU-Präsidentschaft issued in 2006. Thanks to Babsy! ♥