Some of the village of Hinterbrühl was built directly above the stalactite-covered waters of Europe's largest underground lake. From the entrance a few hundred yards from the edge of town, you'll descend a steep flight of stairs before facing the extensively illuminated waters of a shallow, very still, and very cold underground lake. The famous natural marvel was the site of the construction of the world's first jet plane and other aircraft during World War II. Expect a running commentary in German and broken English during the 20-minute boat ride.