On this day we take you to the biggest and worldwide known slovenian cave system the postojna caves a two million years old and 21 km long system of subterranean caves is one of the most easily accessible network of underground caves in the world a ride by electrical train will be followed by a guided walk through a series of cave halls with drop stones pillars and translucent curtains that create unforgettable impressions the underworld system is a home of the mysterious unique and rare