The Rieseneishöhle, or Giant Ice Cave, lies high in the Dachstein massif above the village of Obertraun on the east side of the lake. A cable car climbs in two stages to 1,350 metres, where a 15-minute walk leads to the cave entrance. Inside, walkways and stairs wind past frozen waterfalls and great curved sheets of ice, lit by coloured spotlights and accompanied by classical music — a deliberately theatrical effect that somehow doesn't quite spoil the geology.
The cave is naturally refrigerated by cold air sinking from above; it stays below freezing even in midsummer when the temperature outside is 30°C. Combine the visit with the nearby Mammoth Cave (limestone, not ice), the Five Fingers viewing platform on the same plateau, and a meal at the mountain hut at the cable-car top station. Allow most of a day. Cable car closed mid-October to late April.