The Kristallhöhle in the Kubach district of Weilburg is described as unique in Germany for the density and quality of its calcite crystal formations. The cave extends roughly 200 metres and reaches heights of up to 30 metres in its largest chamber. The walls are covered with crystalline calc-sinter deposits that catch the light in unusual ways, and the underground temperature remains constant year-round.
The cave is open to visitors and guided tours depart regularly during opening hours. It takes about 45 minutes to an hour to walk through with a guide. The cave is located a few kilometres from the town centre and is easily reached by car or bicycle.
Combined with the Schloss, the ship tunnel, and a stretch of the Lahn, the Kristallhöhle rounds out a full day in and around Weilburg. It is the kind of quiet, understated attraction that rewards visitors who make the effort — less famous than the Dechenhöhle or Sauerland caves, but worth the detour.