Exploring the caves - Škocjan

Because of  their extraordinary significance for the world's
  natural heritage, in 1986 the Škocjan Caves were included in
  UNESCO's world heritage list. They were listed as a first underground
  wetland in the world in 1999 representing Ramsar wetlands of
  international importance.
   
  Škocjan Caves represent the most important underground
  phenomenon of Kras region, and are also one of the finest
  caves in the world. Through the ages Reka river has created
  the fabulous world of Karst wonders, such as collapse dolinas,
  natural bridges, subterranean waterfalls, lakes, gorges, rapids, etc.
  
   
  
  One of the reasons for the application of Škocjan Caves
  for the status of the World Heritage Site lies also in its
  huge and marvellous underground chambers. The vastest
  of them is 123 m wide and 300 m long Martelova dvorana
  (Martel's chamber) that reaches at its highest point the stunning
  height of 146 m!
  Standing on the Cerkvenik bridge (see the photo) you may
  admire the underground gorge and the mysterious Reka river
  rushing forward 45 m below your feet.
   
  
  This exceptional system of limestone caves contains collapsed
  dolinas, about 6 kilometres of underground passages, caves more
  than 200 metres deep and many waterfalls. This is one of the
  most famous sites in the world for the study of karstic (limestone)
  phenomena.
  
  From: http://www.park-skocjanske-jame.si/