There’re thousands of caves in Slovenia ranging in size form extensive systems like Postojna or Skocjan to medium-sized caverns. One of the reasons for us being in Slovenia is to visit the larger Škocjan and Postojna Caves. 
Last night we camped at Razdrto between the National Parks and this morning we headed to south a few kilometers to the Škocjan caves then headed North 30k or so to visit the Postojna caves in the afternoon.
Skocjan Caves
The Skocjan Caves are a vast underground canyon straight out of a Jules Vern novel with 
a raging river running through it. This unique underground canyon is a UNESCO listed site and it is no 
wonder. It ranks among the most important caves in the world and represents the most significant underground phenomena.
It is surreal experience walking along the 140 meter high canyon walls completely underground.
Postojna Caves
We moved on to the Postojna Caves. These caves are unusual in that you take an underground train into the cave system. The cave is a 20,570 m long Karst cave system and is the longest cave system in the country. It is the largest cave in the “classic karst” and the most vis¬ited show cave in Europe. In 1872 railway lines were laid in the cave; electricity arrived in 1884.
Today you begin your visit aboard the cave train. They run three long trains at a time and we were surprised at the number of people involved on each tour. The next surprise was how fast the trains went in no time we were wizzing through the cave wondering how we were supposed to see anything at this speed!!
After 2 kilometers the train stopped and we all got off and made our way to staging areas under signs indicating the language to be used for the tour.
We were pleased that the train trip was not the whole tour but the walking cave tour begins at 2 kilometers inside the cave!
I thought I would be cool and wear my sunglasses in the cave!!
Thanks for your short but still interesting info, as I'm about to visit these caves.