Škocjan Caves handed geology one of its everyday words. The stark limestone plateau above them is the Classical Karst — the Kras — and it is from this ground that science borrowed the terms "karst" and "doline" for cave-riddled rock the world over. The caves hold the reason: the Reka River runs for kilometres across the surface, then vanishes at the foot of a cliff and thunders on through the rock, having gouged one of the largest underground river canyons on Earth. Its passages stretch more than six kilometres and drop over 200 metres; the greatest hall, Martel's Chamber, is 308 metres long and 146 metres high — over two million cubic metres of dark, among the biggest cave chambers known. From here the Reka runs unseen for 34 kilometres before rising again near the Adriatic. Named a UNESCO World Heritage Site in 1986, Škocjan became in 1999 the first underground wetland ever granted Ramsar protection.
The signature moment is the Cerkvenik Bridge, a slender span flung across the gorge some 45 metres above the water; the Reka roars in the dark beneath your feet while the passage opens vast on either side. A guided walk leads there through halls of stalactites and stalagmites and past still river pools lit by daylight that falls from a chimney in the rock. Beyond the bridge the trail climbs into two enormous collapsed sinkholes, Velika and Mala dolina — green-walled pits where the river reappears far below and natural stone arches hang overhead. In the flooded inner passages lives the olm, a blind, pale salamander found only in this karst. The cave holds near 12°C all year, so it can be visited in any season, though only on a timed guided tour, with cameras left behind on the main canyon route.