#1. Postojna
The karst cave at Postojna is one of the largest in the world, and its stalagmite and stalactite formations are unequalled anywhere.
Among Slovenia’s most popular attractions, it's a busy spot – the amazing thing is how the large crowds at the entrance seem to get swallowed whole by the size of the cave, and the tourist activity doesn't detract from the wonder.It's a big, slick complex, and it doesn't come cheap.
But it's still worth every minute you can spend in this magical underground world. The adjacent town of Postojna serves as a gateway to the caves and is otherwise a fairly attractive provincial Slovenian town.