When in Slovenia one has to add some caves to the visit list. These are no ordinary caves. Škocjan Cave system is one of the largest known underground canyons in the world; is an example of natural beauty with great aesthetic value; due to particular microclimatic conditions, a special ecosystem has developed; the area has great cultural and historical significance as it has been inhabited since the prehistoric times and is typical example of contact karst. - in: wikipedia
Due to their exceptional significance, the Škocjan Caves were entered on UNESCO’s list of natural and cultural world heritage sites in 1986.
International scientific circles have thus acknowledged the importance of the Caves as one of the natural treasures of planet Earth.
Ranking among the most important caves in the world, the Škocjan Caves represent the most significant underground phenomena in both the Karst region and Slovenia. - in: http://www.park-skocjanske-jame.si/eng/caves.shtml
On the card there's the Cekvenik bridge in Šumeča jama Cave, which crosses the Reka river.
This exceptional system of limestone caves comprises collapsed dolines, some 6 km of underground passages with a total depth of more than 200 m, many waterfalls and one of the largest known underground chambers. The site, located in the Kras region (literally meaning Karst), is one of the most famous in the world for the study of karstic phenomena. - in: https://whc.unesco.org/en/list/390/