Listed as a UNESCO World Heritage Site, these caves were hollowed out by the Reka river, which disappears underground before emerging 34km further on at the source of the Timava. An artificial passage leads to the dry caves, now adorned with concretions. The Great Hall, with its 15m stalagmites and its columns, is impressive, but is eclipsed by the Murmuring Cave. The path emerges 100m above the canyon, carved out by its roaring river.