I went on a Sunday in July 2015 and found the visitor centre thronged with people – mostly Slovenian tourists it seemed. The temperature was in the high 30s, making the caves an attractive refuge. At €16 it isn't cheap, but is worth it. Note that the €35 permit required to drive in Slovenia only seems to apply if you go on the motorway system. If you are coming in from Trieste then that is not necessary (although it we did use it to go to Idrija the next day).

We were led down to the cave’s entrance in a huge group of at least 70 people, so it was a relief when the guides announced we were going to be split into three groups, by language.

Inside the cave one sees a remarkable chamber of stalactites and stalagmites, followed by a descent reminiscent of journeying into the bowels of Mount Doom. The cave opens up into a much larger cavern as the tour proceeds to link up with the underground Reka river (one of the many tautological river names in the world).

After crossing a stunning bridge, the path winds its way out of the caves and into the semi-daylight, where bats swoop from the ceiling, despite being nocturnal. The walk back up to the visitor centre was rewarding, affording us views of the Reka rushing through gulleys and funnels.