The Postojna Cave is an underground system of chambers, halls and passages that are approximately 20.6km long. The cave is two million years old and it was carved out by the Pivka River, which vanishes in an underground tunnel near the cave's entrance. The river continues its way in the ground, hollowing out various caves, and emerges as the Unica River.

Visitors explore about 5.7 kilometres of the cave in an hour and a half tour. Around 4 kilometres of the system is covered by an electric train, which takes you to Velika Gora (Eng. The Big Mountain cavern). Some say it is a trip that is like entering a secret lair of a James Bond villain. From there, a guide accompanies you through halls, chambers and galleries in one of four languages. The Postojna Cave has a temperature of 8°C to 10°C, so a jacket and shoes are essential.

There are dry galleries, decorated with white stalactites shaped like needles, large icicles and even spaghetti. The stalagmites are shapes like pears, cauliflower and sand castles, but there are also bizarre columns, pillars and translucent curtains that look like rashers of bacon.