Found it!

[Lankyman]

In the seventies my parents took us on holiday to (then) Yugoslavia. We stayed at Opatija in what is now Croatia. Not being beach people we travelled around a fair bit and got to see the superb show cave at Postojna. Dad even drove us all to Venice for a day (I well recall the big arguments over map reading!). What made as big an impression on my teenage mind as St Mark's Square was the cave we visited on the way back to Opatija. I can remember driving up a dirt road from the highway following a homemade sign and descending a big open pothole on a path that spiralled down into the depths. Down below were big passages and lots of formations all lit by naked light bulbs in a makeshift fashion. Our guide spoke no English but that wasn't a problem. I thought it was better than Postojna and it inspired me even more to get into 'proper' caving not long after. I was noodling around recently on Google maps to try and find this cave and here it is - Dimnice Jama! Isn't technology wonderful?

Dimnice Cave · Materija

maps.app.goo.gl

You can even tour the entrance pitch on Street view!