One hour tours of 'Hell Cave' take you 1200m from the lower wet area to the upper dry section. Among the highlights, you'll pass a 4m-high waterfall, the Silent Hall with perfect acoustics and the Hall of Fantasy, where stalactites and stalagmites become snakes, pigeons, cauliflower and frogs. It's 10°C in there and very slippery; take a wrap and wear sturdy shoes.
It's roughly 15km northwest from Celje and 4km north of the Šempeter Roman necropolis, on the road heading to Kale.